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)