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/import.c b/Python/import.c
index d79fa18..3bf8fe0 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -9,6 +9,7 @@
 #include "pycore_pyhash.h"
 #include "pycore_pylifecycle.h"
 #include "pycore_pymem.h"
+#include "pycore_interp.h"       // _PyInterpreterState_ClearModules()
 #include "pycore_pystate.h"
 #include "pycore_sysmodule.h"
 #include "errcode.h"