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/db/models/base.pyi

@@ -8,6 +8,7 @@ _Self = TypeVar("_Self", bound="Model")
 
 class Model(metaclass=ModelBase):
     class DoesNotExist(Exception): ...
+    class MultipleObjectsReturned(Exception): ...
     class Meta: ...
     _meta: Any
     _default_manager: Manager[Model]
@@ -15,6 +16,7 @@ class Model(metaclass=ModelBase):
     def __init__(self: _Self, *args, **kwargs) -> None: ...
     def delete(self, using: Any = ..., keep_parents: bool = ...) -> Tuple[int, Dict[str, int]]: ...
     def full_clean(self, exclude: Optional[List[str]] = ..., validate_unique: bool = ...) -> None: ...
+    def clean(self) -> None: ...
     def clean_fields(self, exclude: List[str] = ...) -> None: ...
     def validate_unique(self, exclude: List[str] = ...) -> None: ...
     def save(
@@ -34,6 +36,7 @@ class Model(metaclass=ModelBase):
     ): ...
     def refresh_from_db(self: _Self, using: Optional[str] = ..., fields: Optional[List[str]] = ...) -> _Self: ...
     def get_deferred_fields(self) -> Set[str]: ...
+    def __getstate__(self) -> dict: ...
 
 class ModelStateFieldsCacheDescriptor: ...