install via pip fails on buster: Failed to build vext.pyqt5

$ pip install leap.bitmask[gui]                                                                 
Collecting leap.bitmask[gui]                                                                                    
  Using cached leap.bitmask-0.10.1-py2-none-any.whl                                                             
Collecting cryptography (from leap.bitmask[gui])                                                                
  Using cached cryptography-2.1.4-cp27-cp27mu-manylinux1_i686.whl                                               
Collecting zope.interface (from leap.bitmask[gui])                                                              
  Using cached zope.interface-4.4.3-cp27-cp27mu-manylinux1_i686.whl                                             
Collecting python-daemon (from leap.bitmask[gui])                                                               
  Using cached python_daemon-2.1.2-py2.py3-none-any.whl                                                         
Collecting twisted>=14.0.0 (from leap.bitmask[gui])                                                             
Collecting service-identity (from leap.bitmask[gui])                                                            
  Using cached service_identity-17.0.0-py2.py3-none-any.whl                                                     
Collecting srp (from leap.bitmask[gui])                                                                         
Collecting colorama (from leap.bitmask[gui])                                                                    
  Using cached colorama-0.3.9-py2.py3-none-any.whl                                                              
Collecting certifi (from leap.bitmask[gui])                                                                     
  Using cached certifi-2018.1.18-py2.py3-none-any.whl                                                           
Collecting leap.common (from leap.bitmask[gui])                                                                 
  Using cached leap.common-0.6.2-py2.py3-none-any.whl                                                           
Collecting leap.bitmask-js; extra == "gui" (from leap.bitmask[gui])                                             
Collecting vext.pyqt5; extra == "gui" (from leap.bitmask[gui])                                                  
  Using cached vext.pyqt5-0.5.21.tar.gz                                                                         
Collecting idna>=2.1 (from cryptography->leap.bitmask[gui])                                                     
  Using cached idna-2.6-py2.py3-none-any.whl                                                                    
Collecting cffi>=1.7; platform_python_implementation != "PyPy" (from cryptography->leap.bitmask[gui])           
  Using cached cffi-1.11.5-cp27-cp27mu-manylinux1_i686.whl                                                      
Collecting enum34; python_version < "3" (from cryptography->leap.bitmask[gui])                                  
  Using cached enum34-1.1.6-py2-none-any.whl                                                                    
Collecting six>=1.4.1 (from cryptography->leap.bitmask[gui])                                                    
  Using cached six-1.11.0-py2.py3-none-any.whl                                                                  
Collecting ipaddress; python_version < "3" (from cryptography->leap.bitmask[gui])
  Using cached asn1crypto-0.24.0-py2.py3-none-any.whl                                                           
Collecting setuptools (from zope.interface->leap.bitmask[gui])                                                  
  Using cached setuptools-38.5.1-py2.py3-none-any.whl                                                           
Collecting docutils (from python-daemon->leap.bitmask[gui])                                                     
  Using cached docutils-0.14-py2-none-any.whl                                                                   
Collecting lockfile>=0.10 (from python-daemon->leap.bitmask[gui])                                               
  Using cached lockfile-0.12.2-py2.py3-none-any.whl                                                             
Collecting hyperlink>=17.1.1 (from twisted>=14.0.0->leap.bitmask[gui])                                          
  Using cached hyperlink-18.0.0-py2.py3-none-any.whl                                                            
Collecting constantly>=15.1 (from twisted>=14.0.0->leap.bitmask[gui])
  Using cached constantly-15.1.0-py2.py3-none-any.whl
Collecting incremental>=16.10.1 (from twisted>=14.0.0->leap.bitmask[gui])
  Using cached incremental-17.5.0-py2.py3-none-any.whl
Collecting Automat>=0.3.0 (from twisted>=14.0.0->leap.bitmask[gui])
  Using cached Automat-0.6.0-py2.py3-none-any.whl
Collecting attrs (from service-identity->leap.bitmask[gui])
  Using cached attrs-17.4.0-py2.py3-none-any.whl
Collecting pyasn1-modules (from service-identity->leap.bitmask[gui])
  Using cached pyasn1_modules-0.2.1-py2.py3-none-any.whl
Collecting pyopenssl>=0.12 (from service-identity->leap.bitmask[gui])
  Using cached pyOpenSSL-17.5.0-py2.py3-none-any.whl
Collecting pyasn1 (from service-identity->leap.bitmask[gui])
  Using cached pyasn1-0.4.2-py2.py3-none-any.whl
Collecting jsonschema (from leap.common->leap.bitmask[gui])
  Using cached jsonschema-2.6.0-py2.py3-none-any.whl
Collecting pyzmq>=14.4.1 (from leap.common->leap.bitmask[gui])
  Using cached pyzmq-17.0.0-cp27-cp27mu-manylinux1_i686.whl
Collecting python-dateutil (from leap.common->leap.bitmask[gui])
  Using cached python_dateutil-2.6.1-py2.py3-none-any.whl
Collecting txzmq>=0.7.3 (from leap.common->leap.bitmask[gui])
Collecting vext>=0.5.21 (from vext.pyqt5; extra == "gui"->leap.bitmask[gui])
Collecting pycparser (from cffi>=1.7; platform_python_implementation != "PyPy"->cryptography->leap.bitmask[gui]$
Collecting functools32; python_version == "2.7" (from jsonschema->leap.common->leap.bitmask[gui])
Collecting ruamel.yaml>=0.11.10 (from vext>=0.5.21->vext.pyqt5; extra == "gui"->leap.bitmask[gui])
Collecting ruamel.ordereddict; platform_python_implementation == "CPython" and python_version <= "2.7" (from ru$mel.yaml>=0.11.10->vext>=0.5.21->vext.pyqt5; extra == "gui"->leap.bitmask[gui])
Building wheels for collected packages: vext.pyqt5
  Running setup.py bdist_wheel for vext.pyqt5 ... error
  Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-IHL2_0/vext.pyqt5/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /tmp/tmp49mLpWpip-wheel- --python-tag cp27:
  running bdist_wheel
  running build
  installing to build/bdist.linux-i686/wheel
  running install
  vext.pyqt5 Install
  Not installing PTH file to real prefix
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/tmp/pip-build-IHL2_0/vext.pyqt5/setup.py", line 86, in <module>
      install_requires=[vext_version],
    File "/home/ts/.local/lib/python2.7/site-packages/setuptools/__init__.py", line 129, in setup
      return distutils.core.setup(**attrs)
    File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
      dist.run_commands()
    File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
      self.run_command(cmd)
    File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
      cmd_obj.run()
    File "/usr/lib/python2.7/dist-packages/wheel/bdist_wheel.py", line 257, in run
      self.distinfo_dir)
    File "/usr/lib/python2.7/dist-packages/wheel/bdist_wheel.py", line 393, in egg2dist
      raise ValueError(err)
  ValueError: Egg metadata expected at build/bdist.linux-i686/wheel/vext.pyqt5-0.5.21-py2.7.egg-info but not found
  
  ----------------------------------------
  Failed building wheel for vext.pyqt5
  Running setup.py clean for vext.pyqt5
Failed to build vext.pyqt5
Installing collected packages: idna, pycparser, cffi, enum34, six, ipaddress, asn1crypto, cryptography, setuptools, zope.interface, docutils, lockfile, python-daemon, hyperlink, constantly, incremental, attrs, Automat, twisted, pyasn1, pyasn1-modules, pyopenssl, service-identity, srp, colorama, certifi, functools32, jsonschema, pyzmq, python-dateutil, txzmq, leap.common, leap.bitmask-js, ruamel.ordereddict, ruamel.yaml, vext, vext.pyqt5, leap.bitmask
Speicherzugriffsfehler