Add tests for the free-threaded build

[ngoldbaum]

IMO it's worth adding 3.14t and 3.15t builds along with 3.14 and 3.15:

skeleton/.github/workflows/main.yml

Lines 48 to 50 in 684a315

	- python: "3.14"
	platform: ubuntu-latest
	- python: "3.15"

That allows you to catch issues due to differences on free-threaded builds, even without multithreaded tests.

c.f. pypa/setuptools#5193 where I'm working on PEP 803 support in setuptools and noticed there's no test with a free-threaded interpreter

[jaraco]

Good call! What about 3.13? What about non-linux platforms? I'm thinking we should support it across all platforms and for every stable Python version (so not 3.15 until stable).

[ngoldbaum]

I'd skip 3.13t because cibuildwheel will be dropping 3.13t build support in its next release. https://github.com/pypa/cibuildwheel/releases/tag/v3.4.1