Drop Python 3.8 support

[lemon24]

We can drop 3.8 support, per https://reader.readthedocs.io/en/latest/dev.html#supported-python-versions:

• https://packages.ubuntu.com/jammy/python3 is 3.10.
• PyPy 3.9 is stable, and we already support it from 2.14.

Prerequisites:

• [x] release 3.3 with Python 3.11 support
• [x] migrate my own deployment to the latest Ubuntu LTS

Things reader can take advantage of after dropping 3.8

• typing: generic collections.abc, generic built-in collections (no from future import annotations required)
• dict | and |=
• str.removeprefix()
• zoneinfo (might replace some of the datetime handing functions in reader)
• pkgutil.resolve_name (no more vendoring)

Also, we can remove the "reader works out of the box on Windows only starting with Python 3.9" note from install.rst.

[lemon24]

To do:

• [x] remove from CI / tox
• [x] update setup.cfg
• [x] update docs + changelog
• [x] remove resolve_name
• [x] take advantage of generic builtins and collections.abc (pyupgrade --py39-plus kinda works?)