This addresses a failure in the `py38-dev` tests
caused by markupsafe recently dropping Python 3.8 support.
Changed in this commit:
* Drop Python 3.8 support
* Update CI and tox testing
* Update docs references to Python 3.8
* Remove a Windows / Python 3.8 admonition in the docs
* Address type annotations
* Incorporate style fixes
Still to do:
* `src/flask/sessions.py` contains these comments:
> TODO generic when Python > 3.8
This TODO should now be unblocked.
