Drop support for EOL Python 3.8

[hugovk]

Python 3.8 has been end-of-life since October 2024: https://devguide.python.org/versions/

It looks like some of the current CI failures are due to changes in virtualenv to do with wheel, which is different between 3.8 and 3.9+:

The --no-wheel and --wheel options are deprecated. They have no effect for Python > 3.8 as wheel is no longer bundled in virtualenv.

Dropping 3.8 should make this easier to fix by dealing with just one code path.

This PR drops support by removing 3.8 from the CI and tox.toml, and updating pyproject.toml and docs.

Because the 3.9+ CI is failing, no attempt is made to upgrade any syntax (for example, by bumping Ruff's target-version = "py38"), as that should only be done when the CI is green.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation