Disable assuming Optional type for values with None default (#16029)

https://www.python.org/dev/peps/pep-0484/#union-types
"Type checkers should move towards requiring the optional type to be
made explicit."

homeassistant/auth/providers/insecure_example.py

@@ -98,7 +98,8 @@ class LoginFlow(data_entry_flow.FlowHandler):
         self._auth_provider = auth_provider
 
     async def async_step_init(
-            self, user_input: Dict[str, str] = None) -> Dict[str, Any]:
+            self, user_input: Optional[Dict[str, str]] = None) \
+            -> Dict[str, Any]:
         """Handle the step of the form."""
         errors = {}