Allowing building for freethreaded Python

[ayshih]

It looks like reproject cannot currently be built for freethreaded Python because reproject insists on the Limited API:

reproject/pyproject.toml

Lines 207 to 208 in c3c56d8

	[tool.distutils.bdist_wheel]
	py-limited-api = "cp311"

Can an automatic switch be implemented a la what is done in pyerfa to support freethreading?

ping @Cadair

[astrofrog]

Since we don't have a setup.py, I think a cleaner approach is #600