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.

django-stubs/core/serializers/__init__.pyi

@@ -1,5 +1,4 @@
-from collections import OrderedDict
-from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, Type, Union
+from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, Type, Union, Iterable
 
 from django.apps.config import AppConfig
 from django.db.models.base import Model
@@ -33,5 +32,5 @@ def serialize(
 ) -> Optional[Union[bytes, str]]: ...
 def deserialize(format: str, stream_or_string: Any, **options: Any) -> Union[Iterator[Any], Deserializer]: ...
 def sort_dependencies(
-    app_list: Union[List[Tuple[AppConfig, None]], List[Tuple[str, List[Type[Model]]]]]
+    app_list: Union[Iterable[Tuple[AppConfig, None]], Iterable[Tuple[str, Iterable[Type[Model]]]]]
 ) -> List[Type[Model]]: ...