gh-134009: Expose PyMutex_IsLocked in the public C API (gh-134365)

The `PyMutex_IsLocked()` function is useful in assertions for verifying
that code maintains certain locking invariants.

Python/lock.c

@@ -634,3 +634,11 @@ PyMutex_Unlock(PyMutex *m)
         Py_FatalError("unlocking mutex that is not locked");
     }
 }
+
+
+#undef PyMutex_IsLocked
+int
+PyMutex_IsLocked(PyMutex *m)
+{
+    return _PyMutex_IsLocked(m);
+}