Test `pip` against Python Free Threaded builds

[notatallshaw]

We should start testing Python Free Threaded builds against pip, we don't expect issues as pip doesn't directly take advantage of threading, but that's not to say there can't be issues from vendored or standard libraries.

There are some choices to makes, whether to test: 3.13t, 3.14t, and/or 3.15t to start with. And whether to test on Linux, Mac, and/or Windows.

Also whether to add classifiers, like packaging has: https://github.com/pypa/packaging/pull/1098/changes