bpo-39984: _PyThreadState_DeleteCurrent() takes tstate (GH-19051)

* _PyThreadState_DeleteCurrent() now takes tstate rather than
  runtime.
* Add ensure_tstate_not_null() helper to pystate.c.
* Add _PyEval_ReleaseLock() function.
* _PyThreadState_DeleteCurrent() now calls
  _PyEval_ReleaseLock(tstate) and frees PyThreadState memory after
  this call, not before.
* PyGILState_Release(): rename "tcur" variable to "tstate".