gh-130091: Reorder `_PyThreadState_Attach` to avoid data race (gh-130092)

This moves `tstate_activate()` down to avoid a data race in the free
threading build on the `_PyRuntime`'s thread-local `autoTSSkey`. This
key is deleted during runtime finalization, which may happen
concurrently with a call to `_PyThreadState_Attach`.

The earlier `tstate_try/wait_attach` ensures that the thread is blocked
before it attempts to access the deleted `autoTSSkey`.

This fixes a TSAN reported data race in
`test_threading.test_import_from_another_thread`.