bpo-35081: Cleanup pystate.c and pystate.h (GH-10240)

bpo-35081: Cleanup pystate.c and pystate.h (GH-10240)

* Remove _PyThreadState_Current
* Replace GET_TSTATE() with PyThreadState_GET()
* Replace GET_INTERP_STATE() with _PyInterpreterState_GET_UNSAFE()
* Replace direct access to _PyThreadState_Current with
  PyThreadState_GET()
* Replace _PyThreadState_Current with
  _PyRuntime.gilstate.tstate_current
* Rename SET_TSTATE() to _PyThreadState_SET(), name more
  consistent with _PyThreadState_GET()
* Update outdated comments