fix: Fixed typing for Meta configuration

[noamkush]

This bundles a few (related) fixes into one PR:

1. Meta.exclude is typed as a possible str but that is in no way supported
2. Meta.fields_optional can be a "__all__" but not any other str
3. fields_optional is typed as str but SchemaFactory isn't typed to take an str
4. Missing mypy strict_equality which would have caught the issue in SchemaFactory
5. Old mypy version that is incompatible with recent django-stubs - upgraded to the first supported version
6. @no_type_check in this area - removed and fixed (or ignored) all issues