Support for latest Bugzilla (4.4.3+) and current PyBugz

Bugzilla 4.4.3 dropped the cookie authentication and now uses tokens.
PyBugz was updated to use the tokens but it also dropped support of python 2 some time ago.

This puts gitzilla on python 3 and adds the token authentication required for newer installations.
Tested on Bugzilla 4.4.6