refactor: remove redundant None default

It is unnecessary to provide `None` as the default value when the key is not present in the dictionary as `get` implicitly returns `None`.  Not preferred:

Author: deepsource-autofix[bot]

flask_shell2http/classes.py

@@ -99,7 +99,7 @@ def cleanup_temp_dir(self, future: Future) -> None:
         key: Optional[str] = future.result().get("key", None)
         if not key:
             return None
-        tmpdir: Optional[str] = self.__tmpdirs.get(key, None)
+        tmpdir: Optional[str] = self.__tmpdirs.get(key)
         if not tmpdir:
             return None