gh-121368: Fix seq lock memory ordering in _PyType_Lookup (#121388)

The `_PySeqLock_EndRead` function needs an acquire fence to ensure that
the load of the sequence happens after any loads within the read side
critical section. The missing fence can trigger bugs on macOS arm64.

Additionally, we need a release fence in `_PySeqLock_LockWrite` to
ensure that the sequence update is visible before any modifications to
the cache entry.

Modules/_testcapi/pyatomic.c

@@ -125,6 +125,7 @@ test_atomic_fences(PyObject *self, PyObject *obj) {
     // Just make sure that the fences compile. We are not
     // testing any synchronizing ordering.
     _Py_atomic_fence_seq_cst();
+    _Py_atomic_fence_acquire();
     _Py_atomic_fence_release();
     Py_RETURN_NONE;
 }