`python-version` output for free-threaded build should have t suffix

[ngoldbaum]

Originally suggested by @colesbury over on the PyO3 repo: PyO3/pyo3#4882 (comment)

As you can see from this code search, outputs.python-version is a pretty common cache key. We should help users avoid footguns caused by invalid caches when they introduce free-threaded builds by adding a 't' to the end of the python-version output.