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]

test-data/typecheck/model.test

@@ -0,0 +1,28 @@
+[CASE test_model_subtype_relationship_and_getting_and_setting_attributes]
+from django.db import models
+
+class A(models.Model):
+    pass
+
+class B(models.Model):
+    b_attr = 1
+    pass
+
+class C(A):
+    pass
+
+def service(a: A) -> int:
+    pass
+
+a_instance = A()
+b_instance = B()
+reveal_type(b_instance.b_attr) # E: Revealed type is 'builtins.int'
+
+
+reveal_type(b_instance.non_existent_attribute) # E: Revealed type is 'Any'
+b_instance.non_existent_attribute = 2
+
+service(b_instance)  # E: Argument 1 to "service" has incompatible type "B"; expected "A"
+
+c_instance = C()
+service(c_instance)

test-data/typecheck/model_create.test

@@ -27,12 +27,16 @@ class Parent1(models.Model):
 class Parent2(models.Model):
     id2 = models.AutoField(primary_key=True)
     name2 = models.CharField(max_length=50)
+
+# TODO: Remove the 2 expected errors on the next line once mypy issue https://github.com/python/mypy/issues/2619 is resolved:
 class Child1(Parent1, Parent2):
     value = models.IntegerField()
 class Child4(Child1):
     value4 = models.IntegerField()
 Child4.objects.create(name1='n1', name2='n2', value=1, value4=4)
 [out]
+main:10: error: Definition of "objects" in base class "Parent1" is incompatible with definition in base class "Parent2"
+main:10: error: Definition of "_default_manager" in base class "Parent1" is incompatible with definition in base class "Parent2"
 
 [CASE optional_primary_key_for_create_is_error]
 from django.db import models