add CI builds for free-threaded Python versions

[ngoldbaum]

I noticed there weren't any free-threaded builds for #149.

[ngoldbaum]

I'm confused why there's a complaint about an uninitialized variable, but only on 3.13t under GCC apparently:

test_pythoncapi_compat_cext.c: In function ‘test_long_api’:
test_pythoncapi_compat_cext.c:1439:26: error: ‘digits’ may be used uninitialized [-Werror=maybe-uninitialized]
 1439 |     ((digit *)digits)[0] = 123;
      |     ~~~~~~~~~~~~~~~~~~~~~^~~~~
test_pythoncapi_compat_cext.c:1431:11: note: ‘digits’ was declared here
 1431 |     void *digits;
      |           ^~~~~~

@vstinner do you have any idea what's going wrong?

[ngoldbaum]

It looks like the missing error checking was tripping up the compiler, but only on 3.13t. Not sure why, but adding error checking fixes it. This is ready now, I think.