AttributeError: 'module' object has no attribute 'OP_NO_TLSv1_1'

Installing bitmask with pip, ibitmask fails with: AttributeError: 'module' object has no attribute 'OP_NO_TLSv1_1'. Downgrading Twisted-17.9.0 to 13.1.0 solves the issue.

Traceback (most recent call last):                                                            
  File "/home/user/bin/bitmask", line 7, in <module>                                                       
    from leap.bitmask.gui.app import start_app                                                        
  File "/home/user/.local/lib/python2.7/site-packages/leap/bitmask/__init__.py", line 4, in <module>
    from .util import here                                                                            
  File "/home/user/.local/lib/python2.7/site-packages/leap/bitmask/util.py", line 26, in <module>
    from leap.common.files import which                                                                          
  File "/home/user/.local/lib/python2.7/site-packages/leap/common/__init__.py", line 6, in <module>
    from leap.common import events                                                                         
  File "/home/user/.local/lib/python2.7/site-packages/leap/common/events/__init__.py", line 41, in <module>
    from leap.common.events import client
  File "/home/user/.local/lib/python2.7/site-packages/leap/common/events/client.py", line 56, in <module>
    from leap.common.events.server import EMIT_ADDR
  File "/home/user/.local/lib/python2.7/site-packages/leap/common/events/server.py", line 23, in <module>
    import txzmq
  File "/home/user/.local/lib/python2.7/site-packages/txzmq/__init__.py", line 4, in <module>
    from txzmq.connection import ZmqConnection, ZmqEndpoint, ZmqEndpointType
  File "/home/user/.local/lib/python2.7/site-packages/txzmq/connection.py", line 11, in <module>
    from twisted.internet import reactor
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/reactor.py", line 38, in <module>
    from twisted.internet import default
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/default.py", line 56, in <module>
    install = _getInstallFunction(platform)
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/default.py", line 44, in _getInstallFunction
    from twisted.internet.epollreactor import install
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/epollreactor.py", line 24, in <module>
    from twisted.internet import posixbase
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/posixbase.py", line 18, in <module>
    from twisted.internet import error, udp, tcp
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 28, in <module>
    from twisted.internet._newtls import (
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/_newtls.py", line 21, in <module>
    from twisted.protocols.tls import TLSMemoryBIOFactory, TLSMemoryBIOProtocol
  File "/home/user/.local/lib/python2.7/site-packages/twisted/protocols/tls.py", line 63, in <module>
    from twisted.internet._sslverify import _setAcceptableProtocols
  File "/home/user/.local/lib/python2.7/site-packages/twisted/internet/_sslverify.py", line 38, in <module>
    TLSVersion.TLSv1_1: SSL.OP_NO_TLSv1_1,
AttributeError: 'module' object has no attribute 'OP_NO_TLSv1_1'