MAINT: improve support for free-threaded CPython

[rgommers]

As noted in https://github.com/pypa/packaging/issues/727, free-threaded CPython does not support the limited API. Hence raise an exception if a limited-api build is requested.

Also skip/update a few tests that were failing when run under a free-threaded interpreter.

Note that we are already producing the correct wheel tags, since that comes from sysconfig.get_config_var('EXT_SUFFIX').

Addresses part of gh-499. There will be more to do - most importantly adding a CI job. This probably only makes sense to do once the setup-python action adds support (xref https://github.com/actions/setup-python/issues/771). This may happen around the CPython 3.13 beta 1 release (May 7th).

I'll add notes on how to test with a free-threaded build on gh-499 (it's not completely trivial yet).