missing python-service-identity
guido reports this on buster packages:
0: UserWarning: You do not have a working installation of the service_identity module: 'cannot import name opentype'. Please install it from <https://pypi.python.org/pypi/service_identity> and make sure all of its dependencies are satisfied. Without the service_identity module, Twisted can perform only rudimentary TLS client hostname verification. Many valid certificate/hostname mappings may be rejected.
package: 0.10.0+020171006191720.22288+0.10.1+buster0.g1176dacf (/var/lib/dpkg/status) he then upgraded to the 'master' one and still got that.
(note that the 0.10.0 is misreported because the debian changelog hadn't been updated for 0.10.2)