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/config_entries.py

@@ -302,7 +302,7 @@ class ConfigEntries:
         return result
 
     @callback
-    def async_entries(self, domain: str = None) -> List[ConfigEntry]:
+    def async_entries(self, domain: Optional[str] = None) -> List[ConfigEntry]:
         """Return all entries or entries for a specific domain."""
         if domain is None:
             return list(self._entries)