add clean django-stubs folder, add some tests

mypy_django_plugin/plugins/postgres_fields.py

@@ -0,0 +1,11 @@
+from mypy.plugin import Plugin, ClassDefContext
+
+
+def determine_type_of_array_field(context: ClassDefContext) -> None:
+    pass
+
+
+class PostgresFieldsPlugin(Plugin):
+    def get_base_class_hook(self, fullname: str
+                            ):
+        return determine_type_of_array_field