PyLong_GetSign will be added in 3.14a0

[hugovk]

Follow on from #99.

PyLong_GetSign will be added in 3.14.0a0:

• gh-116560: Add PyLong_GetSign() public function cpython#116561

I was getting an error building with latest main:

      In file included from src/encode.c:28:
      src/thirdparty/pythoncapi_compat.h:1285:1: error: static declaration of 'PyLong_GetSign' follows non-static declaration
      PyLong_GetSign(PyObject *obj, int *sign) {
      ^
      /Users/hugo/github/python/cpython/main/Include/cpython/longobject.h:66:17: note: previous declaration is here
      PyAPI_FUNC(int) PyLong_GetSign(PyObject *v, int *sign);
                      ^
      1 error generated.

Fix the version check and comment.

[vstinner]

Merged, thank you.