TST: enable faster coverage in Python>=3.12

[neutrinoceros]

Description

Since version 7.4.2, coverage now fallback gracefully when the option isn't available (i.e. on Python 3.11 and older). This feature is still documented as experimental (as of version 7.5.1), but the migration cost is so low that it seems worth a shot.
Currently it only affects one regular tox env, but the gain (~40%) is significant.

Fixes #15975

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[neutrinoceros]

I guess the real constraint would be !=7.4.0,!=7.4.1 which are the only two versions that know about the environment variable but don't have a graceful fallback mechanism for it, but this seemed tidier.

[neutrinoceros]

Alas, gracefully falling back still means that a warning is emitted, which is enough to fail jobs running coverage with older Pythons. Let me draft this and fix for now. I'll fix it later.

[mhvk]

Alas, gracefully falling back still means that a warning is emitted, which is enough to fail jobs running coverage with older Pythons. Let me draft this and fix for now. I'll fix it later.

Really nice speed-up! We can just ignore the warning perhaps? Alternatively, pin coverage to <7.4 for lower python?

[neutrinoceros]

pin coverage to <7.4 for lower python?

This was my solution until I realized I could simply use the env var in py312 envs only. Now this is stable and doesn't prevent upgrades for any Python.

[mhvk]

OK, happy to hear the warning was already solved by ensuring the option is only set for python 3.12.

This is a bit far from my expertise, but this seems straightforward. In follow-up, we perhaps should use coverage only on python 3.12...