Ref https://bugs.launchpad.net/ubuntu/+source/python3.5/+bug/1650202 and https://github.com/python/typing/commit/7baf6fe2a6d96ab09c807aac174650baa35d15dd
Optional returns a Union, which can't be subclassed.
Ref https:/ /bugs.launchpad .net/ubuntu/ +source/ python3. 5/+bug/ 1650202 and https:/ /github. com/python/ typing/ commit/ 7baf6fe2a6d96ab 09c807aac174650 baa35d15dd
Optional returns a Union, which can't be subclassed.