[3.13] gh-135871: Fix needless spinning in _PyMutex_LockTimed with zero timeout (gh-135872)

Misc/NEWS.d/next/Core and Builtins/2025-06-23-18-08-32.gh-issue-135871.50C528.rst

@@ -0,0 +1,2 @@
+Non-blocking mutex lock attempts now return immediately when the lock is busy
+instead of briefly spinning in the :term:`free threading` build.

Python/lock.c

@@ -58,7 +58,7 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
             return PY_LOCK_ACQUIRED;
         }
     }
-    else if (timeout == 0) {
+    if (timeout == 0) {
         return PY_LOCK_FAILURE;
     }