Add support for Python 3.15

[hugovk]

The Python 3.15 beta is out. 🚀

The 3.15 release manager (👋 hi, that's me!) said:

We strongly encourage maintainers of third-party Python projects to test with 3.15 during the beta phase and report issues found to the Python bug tracker as soon as possible. While the release is planned to be feature-complete entering the beta phase, it is possible that features may be modified or, in rare cases, removed up until the start of the release candidate phase (2026-08-04). Our goal is to have no ABI changes after beta 4 and as few code changes as possible after the first release candidate. To achieve that, it will be extremely important to get as much exposure for 3.15 as possible during the beta phase.

This includes creating pre-release wheels for 3.15, as it helps other projects to do their own testing. However, we recommend that your regular production releases wait until 3.15.0rc1, to avoid the risk of ABI breaks.

Also declare support for 3.13-3.15 with Trove classifiers.

And includes a suggestion: stop testing free-threaded 3.13t. It was experimental, 3.14t and up are properly supported and have better performance, and many other projects are stopping testing 3.13t, such as cibuildwheel and Pillow.

[Copilot]

Pull request overview

Adds CI coverage and packaging metadata updates to recognize and exercise Python 3.15 (including free-threaded builds) during the beta period, aligning GitPython’s declared support with current upstream Python testing guidance.

Changes:

• Add Trove classifiers for Python 3.13–3.15 in setup.py.
• Extend the GitHub Actions test matrix to run on Python 3.15 / 3.15t and stop testing 3.13t.
• Mark 3.15 / 3.15t as “experimental” to enable actions/setup-python pre-release installs via allow-prereleases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
setup.py	Adds Trove classifiers for Python 3.13–3.15 to reflect supported versions.
.github/workflows/pythonpackage.yml	Updates CI matrix to add 3.15/3.15t and remove 3.13t, enabling prereleases for 3.15 via allow-prereleases.

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[Byron]

@EliahKagan I wonder if this comment is still valid? We have been running on Windows for a while, even though maybe not with all tests enabled.

My hope would be that with the improvements to deterministic destruction, more tests can work as filehandles will be closed more reliably.

[Byron]

What an honor, thanks a lot for setting this up for us!

And of course, good to see that it just works!