After that, it is expected that security updates (source only) will be released until 5 years after the release of 3.8 final, so until approximately October 2024. - https://peps.python.org/pep-0569/

### Tasks
- [ ] #1896
- [ ] #1912
- [x] update pyproject.toml to specify python 3.9
- [x] use dict[...] instead of Typing.Dict[...], etc.