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/preconfig.c b/Python/preconfig.c
index 89a6227..db32875 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -1,8 +1,9 @@
 #include "Python.h"
-#include "pycore_initconfig.h"
 #include "pycore_getopt.h"
-#include "pycore_pystate.h"   /* _PyRuntime_Initialize() */
-#include <locale.h>       /* setlocale() */
+#include "pycore_initconfig.h"
+#include "pycore_pymem.h"      // _PyMem_GetAllocatorName()
+#include "pycore_pystate.h"    // _PyRuntime_Initialize()
+#include <locale.h>            // setlocale()
 
 
 #define DECODE_LOCALE_ERR(NAME, LEN) \