Add CPython 3.13 to CI when released (tracker issue)

[neutrinoceros]

I ran some experiments on my fork to test pyerfa against CPython 3.13. I report that current wheels are already compatible with the GIL-enabled build for 3.13.0b2 (which is expected with abi3 wheels, restricted to Python's limited API).
update: #153

However, the free-threaded build isn't compatible yet. Attempting to build pyerfa will explicitly fail with the following error

In file included from erfa/ufunc.c:14:
        /opt/_internal/cpython-3.13.0b1-nogil/include/python3.13t/Python.h:51:4: error: #error "The limited API is not currently supported in the free-threaded build"

Here's the upstream issue to wait on before we can move forward python/cpython#111506

[neutrinoceros]

Update:
python/cpython#111506 (comment) clarifies that free-threading + limited-api compatibility will not land in Python 3.13. I suspect we might want to start experimenting with astropy sooner than 3.14.0b1, so I'm now looking for a way to disable limited-api at build time (it should still be on by default) so we can at least build pyerfa from source with free-threading whenever we're ready for it.

[neutrinoceros]

This issue should be closed when CPython 3.13 (final) is added to regular CI.

[mhvk]

I think #160 was the last bit in adding CPython 3.13 support, so closing this.