py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/objenumerate.c b/py/objenumerate.c
index 2fdf30b..3741446 100644
--- a/py/objenumerate.c
+++ b/py/objenumerate.c
@@ -27,8 +27,8 @@
 #include <stdlib.h>
 #include <assert.h>
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 #include "qstr.h"
 #include "obj.h"
 #include "runtime.h"