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.
diff --git a/Python/initconfig.c b/Python/initconfig.c
index e63d6f6..43e0ccb 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -3,6 +3,7 @@
 #include "pycore_fileutils.h"
 #include "pycore_getopt.h"
 #include "pycore_initconfig.h"
+#include "pycore_interp.h"    // _PyInterpreterState.runtime
 #include "pycore_pathconfig.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pylifecycle.h"