Import Literal from typing_extensions for 3.7 compatibility (#889)

django-stubs/core/exceptions.pyi

@@ -1,6 +1,7 @@
-from typing import Any, Dict, Iterator, List, Literal, Mapping, Optional, Tuple, Union
+from typing import Any, Dict, Iterator, List, Mapping, Optional, Tuple, Union
 
 from django.forms.utils import ErrorDict
+from typing_extensions import Literal
 
 class FieldDoesNotExist(Exception): ...
 class AppRegistryNotReady(Exception): ...

django-stubs/db/migrations/operations/special.pyi

@@ -1,7 +1,8 @@
-from typing import Any, Callable, Dict, List, Literal, Mapping, Optional, Sequence, Tuple, Union
+from typing import Any, Callable, Dict, List, Mapping, Optional, Sequence, Tuple, Union
 
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.migrations.state import StateApps
+from typing_extensions import Literal
 
 from .base import Operation