[PATCH] pl: fix can not build free-thread for plpython extension like 3.14t

This patch fix can not build plpython for free-thread python like python3.14t
which 3.14t is maintain in cpython team

for the doc: https://github.com/python/cpython/blob/1857a40807daeae3a1bf5efb682de9c9ae6df845/Doc/howto/free-threading-extensions.rst#building-extensions-for-the-free-threaded-build

it needs to open the py_limited_api for free-thread build

Thanks
Hong Yi