gh-140061: Use `_PyObject_IsUniquelyReferenced()` to check if objects are uniquely referenced (gh-140062)

The previous `Py_REFCNT(x) == 1` checks can have data races in the free
threaded build. `_PyObject_IsUniquelyReferenced(x)` is a more conservative
check that is safe in the free threaded build and is identical to
`Py_REFCNT(x) == 1` in the default GIL-enabled build.