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

test-data/typecheck/db/models/test_init.yml

@@ -0,0 +1,26 @@
+-   case: field_to_many_and_to_one_attrs_bool_or_none_in_field_base_class
+    main: |
+        from django.db.models import Field
+
+        field: Field
+        my_bool: bool
+
+        my_bool = field.one_to_many
+        my_bool = field.one_to_one
+        my_bool = field.many_to_many
+        my_bool = field.many_to_one
+
+        # Narrowing the types should give us bool
+        assert field.one_to_many is not None
+        my_bool = field.one_to_many
+        assert field.one_to_one is not None
+        my_bool = field.one_to_one
+        assert field.many_to_many is not None
+        my_bool = field.many_to_many
+        assert field.many_to_one is not None
+        my_bool = field.many_to_one
+    out: |
+        main:6: error: Incompatible types in assignment (expression has type "Optional[bool]", variable has type "bool")
+        main:7: error: Incompatible types in assignment (expression has type "Optional[bool]", variable has type "bool")
+        main:8: error: Incompatible types in assignment (expression has type "Optional[bool]", variable has type "bool")
+        main:9: error: Incompatible types in assignment (expression has type "Optional[bool]", variable has type "bool")