add import

Author: tdamsma

sqlalchemy-stubs/dialects/postgresql/base.pyi

@@ -1,7 +1,7 @@
 from ... import schema
 from ...engine import default, reflection
 from ...sql import compiler, expression, sqltypes, type_api
-from typing import Any, Optional, Set, Type, Text, Pattern, Dict, TypeVar, overload
+from typing import Any, Optional, Set, Type, Text, Pattern, Dict, TypeVar, overload, Literal
 from datetime import timedelta
 import uuid