TST: add weekly compatibility checks for CPython 3.15

[neutrinoceros]

Python 3.15 is still early in its development and I don't know that there's a precedent for adding "support" quite this early in the cycle, but given there are 0 failing tests at the moment, I figure it would might still be useful so:

• later breaking changes are detected soon-ish
• this would help any downstream package tested with tox knowing they can try out 3.15 pre-releases safely (or at least that issues might already be known here).

Anyway, I think this is worth a shot. Let me know if this is too early.

• 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
• [N/A] updated/extended the documentation

[gaborbernat]

I'm always reluctant to add this so early because there's no easy way for the check to fail but not fail the entire CI 🤔 would want to have that before we commit to this.

[neutrinoceros]

It could also run as a weekly cron job instead of regular CI. What do you think ?

[gaborbernat]

That would work better 🤔