bpo-36854: Clear the current thread later (GH-17279)

Clear the current thread later in the Python finalization.

* The PyInterpreterState_Delete() function is now responsible
  to call PyThreadState_Swap(NULL).
* The tstate_delete_common() function is now responsible to clear the
  "autoTSSKey" thread local storage and it only clears it once the
  thread state is fully cleared. It allows to still get the current
  thread from TSS in tstate_delete_common().