[3.14] pythongh-143880: Fix data race in functools.partial in free threading build (pythonGH-143882)

pythongh-143880: Fix data race in `functools.partial` in free threading build

The assignment to `pto->vectorcall` isn't thread-safe in the free
threading build. Note that this is already fixed in the main branch.
(cherry picked from commit 0042384)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Author: colesbury

Misc/NEWS.d/next/Library/2026-01-15-13-03-22.gh-issue-143880.sWoLsf.rst

@@ -0,0 +1 @@
+Fix data race in :func:`functools.partial` in the :term:`free threading` build.

Modules/_functoolsmodule.c

@@ -220,7 +220,9 @@ partial_vectorcall_fallback(PyThreadState *tstate, partialobject *pto,
                             PyObject *const *args, size_t nargsf,
                             PyObject *kwnames)
 {
+#ifndef Py_GIL_DISABLED
     pto->vectorcall = NULL;
+#endif
     Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
     return _PyObject_MakeTpCall(tstate, (PyObject *)pto,
                                 args, nargs, kwnames);