Missing stubs (#467)

* adding missing db.backends typings and updated test

* added missing stubs

* reformatted files

* removed contrib.postgres.forms

* fixed test

django-stubs/contrib/gis/db/backends/mysql/schema.pyi

@@ -0,0 +1,16 @@
+from django.db.backends.mysql.schema import DatabaseSchemaEditor as DatabaseSchemaEditor
+from typing import Any
+
+logger: Any
+
+class MySQLGISSchemaEditor(DatabaseSchemaEditor):
+    sql_add_spatial_index: str = ...
+    sql_drop_spatial_index: str = ...
+    geometry_sql: Any = ...
+    def __init__(self, *args: Any, **kwargs: Any) -> None: ...
+    def skip_default(self, field: Any): ...
+    def column_sql(self, model: Any, field: Any, include_default: bool = ...): ...
+    def create_model(self, model: Any) -> None: ...
+    def add_field(self, model: Any, field: Any) -> None: ...
+    def remove_field(self, model: Any, field: Any) -> None: ...
+    def create_spatial_indexes(self) -> None: ...