While converting Djblets to housekeeping, we hit an interesting case

where we needed to allow a deprecated base class to remain a base class

for now, but to warn when otherwise used. Furthermore, the base class

was for deprecation warnings, and needed to use a versioned warning

class when otherwise subclassed.
To handle that, two additions were made:


Subclasses of a ClassDeprecatedMixin or ClassMovedMixin can now

   set housekeeping_skip_warning = True to avoid a warning being

   emitted.


All warning_cls parameters can now take a callable that returns a

   warning class when it's time to emit. This can help avoid circular

   reference issues in this particular type of case.

Unit tests pass.
Tested this with upcoming work on moving Djblets to housekeeping.