Adapt python39
This updates default CI to run with python/3.9, and replaces deprecated imports from typing module based on https://peps.python.org/pep-0585/#implementation
The from __future__ import annotations
imports are still required for Postponed Evaluation of Annotations (https://peps.python.org/pep-0563/) and can't be removed. But this makes it possible to also adapt a new style for Optional and Union (from python/3.10)
Fix #124 (closed)
Edited by wakonig_k