Support Python 3.15

I am opening this issue to track supporting and testing against Python 3.15 upstream. This week, we have started rebuilding our packages for Python 3.15 in Fedora. I will leave any issues we find during the process in this ticket. Thanks!

- [ ] https://github.com/beartype/beartype/issues/656 is needed by spdx-tools. This was fixed in beartype's main branch and downstream in Fedora, but beartype has not cut a new release with the fix yet.
- [ ] Some tests use `commoncode.system.py314` to address changes made in Python 3.14. But this variable checks for `python == 3.14`, not `python >= 3.14`, so some of the tests that broke after the Python 3.14 update started breaking again with 3.15. I fixed this with https://src.fedoraproject.org/rpms/python-commoncode/blob/8a7034a15391da262e23627d39c0b62cf585df4d/f/0001-Make-py314-constant-also-apply-to-py315.patch but not sure if upstream would prefer a different approach.