bpo-40268: Include explicitly pycore_interp.h (GH-19505)

pycore_pystate.h no longer includes pycore_interp.h:
it's now included explicitly in files accessing PyInterpreterState.

Python/codecs.c

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