Avoid using 3.8 in tests when feasible (#13680)
See https://github.com/astral-sh/uv/issues/13676
Python 3.8 is EOL, and we should avoid using it for tests unless we're
covering 3.8-specific behaviors.
In some cases, the Python version we require for the test is arbitrary,
for which I've added a new constant (set to 3.12). In other cases, we
require an older Python version for compatibility with various packages,
like `setuptools`. For those, we can _probably_ switch to a newer
version but it'd be more invasive as we'd need to change our constraints
or `exclude-newer`.
Adds a feature flag for cases where we still need to use the EOL
version.
There's a lot of usage in packse scenarios too, I'll update those
separately because the diff will be large.
