52/model subtypes dont typecheck (#55)

* Fix problem where Model instancess are not considered subtypes of each other due to fallback_to_any = True. Fixes #52.

- Added a stub for __getstate__ to Model.
- Added a stub for clean() to Model.
- Correct arg type for sort_dependencies so they are covariant (Iterable rather than List).

Test ignores:
- Added some test ignores in cases where a model inherits from 2 different base models.
- Added some test ignores for cases that MyPy flags as errors due to variable redefinitions or imports that are incompatible types.

* Address review comment.

test-data/typecheck/managers.test

@@ -21,7 +21,7 @@ from django.db import models
 class MyModel(models.Model):
     authors = models.Manager[MyModel]()
 reveal_type(MyModel.authors)  # E: Revealed type is 'django.db.models.manager.Manager[main.MyModel]'
-reveal_type(MyModel.objects)  # E: Revealed type is 'Any'
+MyModel.objects  # E: "Type[MyModel]" has no attribute "objects"
 [out]
 
 [CASE test_model_objects_attribute_present_in_case_of_model_cls_passed_as_generic_parameter]