Seegitpython-developers#2005. Right now, this does not limit by operating system, butthat is just to verify that there are no OS-specific 3.13t problemswe should know about right now; once that is verified, the macOSand Windows jobs will be removed (excluded) for the time being.The 3.13t jobs added here use `Quansight-Labs/setup-python`, not`actions/setup-python`. The latter also has the ability to use3.13t sinceactions/python-versions#319andactions/setup-python#973 (see alsoactions/setup-python#771), but no versiontag includes this feature yet. It can be used by using `@main` or`@...` where `...` is an OID. The former would risk pulling inother untested features we're no trying to test with, while thelatter would not be easy to upgrade automatically as what we havenow (we would be deliberately keeping a hash not at any tag that isalready not the latest hash on any branch). In contrast, the`Quansight-Labs/setup-python` fork adds this feature while stayingup to date with others. When `actions/setup-python` has a release(stable or prerelease) with this feature, we can switch to it.This could probably be done with less code duplication by using amatrix variable for the action to use. Instead, the "Set up Python"step is split in two, with opposite `if` conditions, so that eachis capable of being recognized and upgraded by Dependabot if a newmajor version is released (in case this ends up remaining in placelonger than expected).

For now, this omits macOS and Windows from the 3.13t ("threaded")tests.The plan ingitpython-developers#2005 is to start without them, and no OS-specificproblems have been identified so far. In particular, in theprevious commit that adds 3.13t without excluding any operatingsystems, all tests in the macOS job passed as expected, and theWindows job had the same failure with the same message as ingitpython-developers#1955,with no XFAIL changed to XPASS (which, if present, would suggestGC differences meriting further exploration of 3.13t on Windows).