gh-135871: Fix needless spinning in _PyMutex_LockTimed (timeout==0)

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

@@ -0,0 +1,2 @@
+Free-threaded (no-GIL) builds: a non-blocking mutex lock attempt now returns
+immediately when the lock is busy instead of briefly spinning.

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;
     }