bpo-30860: Consolidate stateful runtime globals. (#3397)

* group the (stateful) runtime globals into various topical structs
* consolidate the topical structs under a single top-level _PyRuntimeState struct
* add a check-c-globals.py script that helps identify runtime globals

Other globals are excluded (see globals.txt and check-c-globals.py).

Python/codecs.c

@@ -9,6 +9,7 @@ Copyright (c) Corporation for National Research Initiatives.
    ------------------------------------------------------------------------ */
 
 #include "Python.h"
+#include "internal/pystate.h"
 #include "ucnhash.h"
 #include <ctype.h>