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* Update outdated comments
diff --git a/Python/ceval.c b/Python/ceval.c
index 9a96dc7..6443123 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -188,8 +188,7 @@
        We therefore avoid PyThreadState_GET() which dumps a fatal error
        in debug mode.
     */
-    drop_gil((PyThreadState*)_Py_atomic_load_relaxed(
-        &_PyThreadState_Current));
+    drop_gil(PyThreadState_GET());
 }
 
 void