Add "one_to_many", "one_to_one", "many_to_many," and "many_to_one" attributes to Field base class (#449)

* Add "_to_many" and "_to_one" bool attributes to Field base class.

* Remove unused import from test case.

django-stubs/db/models/fields/__init__.pyi

@@ -56,6 +56,10 @@ class Field(RegisterLookupMixin, Generic[_ST, _GT]):
     remote_field: Field
     is_relation: bool
     related_model: Optional[Type[Model]]
+    one_to_many: Optional[bool] = ...
+    one_to_one: Optional[bool] = ...
+    many_to_many: Optional[bool] = ...
+    many_to_one: Optional[bool] = ...
     max_length: int
     model: Type[Model]
     name: str