This change adds typing information for the authentication backends, in

preparation for upcoming work. It codifies the types and arguments

required for any overridden functions, helping to ensure consistent type

signatures for any subclasses.
In the process, some issues were found with the implementations:


BaseAuthBackend.authenticate() doesn't require any specific

   credentials, but subclasses do. Those parameters were not made

   optional in the subclasses, and that could lead to crashes calling

authenticate() with wrong credentials. Now they're optional, and

   error information is logged instead.


We were very inconsistent on whether HttpRequest objects were

   optional or defaulted to None. We now follow Django's lead on most

   of these.


Some code assumed the existence of cetain modules or state that could

   have theoretically been None. We now assert these, helping the type

   checkers.


Some minor code was rewritten to be more type-safe in the process, and

some legacy code was modernized for Python 3.

Unit tests pass.