Get all model bases from runtime context

[mkurnikov]

Fixes #68
@pirate Try this and let me know if it helps.

TODO:

• Look for model bases in the get_base_class_hook using this method
• Tests

[mkurnikov]

@pirate Does the emoji mean that this PR worked for you?

[pirate]

Emoji was from reading the code earlier and seeing that it looked correct superficially, but I just tested it now and it worked! Thanks so much!

[mkurnikov]

@msullivan Back then, you've asked about which benefits django-stubs gets from depending on Django runtime.
This particular issue #68 could be solved with three different solutions:

1. Add type annotations to all third-party libs with custom base models.
2. This hack Requiring field annotations even though type can be inferred from field type #68 (comment)
3. And solution presented in this PR, which just reads all model base classes from runtime.