bpo-40268: Rename _PyInterpreterState_GET_UNSAFE() (GH-19509)

Rename _PyInterpreterState_GET_UNSAFE() to _PyInterpreterState_GET()
for consistency with _PyThreadState_GET() and to have a shorter name
(help to fit into 80 columns).

Add also "assert(tstate != NULL);" to the function.

Python/pystate.c

@@ -661,7 +661,7 @@ PyObject*
 PyState_FindModule(struct PyModuleDef* module)
 {
     Py_ssize_t index = module->m_base.m_index;
-    PyInterpreterState *state = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *state = _PyInterpreterState_GET();
     PyObject *res;
     if (module->m_slots) {
         return NULL;