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: 1. `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. 2. We were very inconsistent on whether `HttpRequest` objects were optional or defaulted to `None`. We now follow Django's lead on most of these. 3. 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.