use typing_extension instead (#17174)

(cherry picked from commit aa0f4d6c4f)

superset/config.py

@@ -38,6 +38,7 @@ from dateutil import tz
 from flask import Blueprint
 from flask_appbuilder.security.manager import AUTH_DB
 from pandas.io.parsers import STR_NA_VALUES
+from typing_extensions import Literal
 from werkzeug.local import LocalProxy
 
 from superset.jinja_context import BaseTemplateProcessor