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

* Move the timeout == 0 guard outside the else so a non-blocking call
  returns immediately after a failed CAS instead of entering the spin loop.
* Reload v on every spin iteration, allowing timed/blocking callers to
  notice an unlock promptly.

No-GIL builds now honor the semantics of non-blocking attempts and avoid
wasted CPU; GIL builds are unaffected (MAX_SPIN_COUNT == 0).

Author: jtibbertsma

Misc/NEWS.d/next/Core and Builtins/135871.mutex-spin.rst

@@ -0,0 +1 @@
+bpo-135871: _PyMutex_LockTimed() no longer spins or fails for timeout==0 in no-GIL builds; the spin loop now reloads the lock state.

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;
     }
 
@@ -89,6 +89,7 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
             // Spin for a bit.
             _Py_yield();
             spin_count++;
+            v = _Py_atomic_load_uint8_relaxed(&m->_bits);
             continue;
         }