Pin test dependencies to fix CI, drop EOL and add Django 6.0

[janbrasna]

Development versions of httpx 1.x branch resolved here directly and also via testing dependencies break some expectations. This pins the version used to the 0.x branch, and also pins sanic-testing to a version with similar constraint to avoid their incompatibility later on, that's ticketed and open on their side.

Something from the test dependencies stopped supporting Python 3.8 without correctly specifying that in packaging, so e.g. a version of fakeredis resolved ends up erroring out. Instead of trying to hunt down a patch or a version constraint and keep chasing similar issues over time, I'd say the more sane thing to do is to stop running the CI and tox matrix on Python 3.8 that's been EOL for some time.

Refreshing the versions, I'm also adding Django 6.0 and removing 3.2 and 5.1 from the matrix being EOLed for a while now, covered by later active LTS.

(Not adding Python 3.14+ at this moment, as the pytest 6.x resolved is not compatible, and some 8.x would need to be used instead — not sure where /what plugin/ the constraint comes from.)

This adds another ruff exclusion from default lint rules that landed with updates, to unblock CI:

• unused-unpacked-variable
  • "RUF059", https://docs.astral.sh/ruff/rules/unused-unpacked-variable/

(Marked up FIXME for now, to tidy up separately if anyone has an opinion…)

Bumps Python versions for linting, packaging and releasing to align with the lowest tested in CI.
Bumps Python version for chores to runner defaults to speed up provisioning from toolcache.

Updates some metadata, mostly aligning python_requires with claimed support.

Also updates actions/* versions for better defaults.