py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/parsenum.c b/py/parsenum.c
index 36b0690..b9801ab 100644
--- a/py/parsenum.c
+++ b/py/parsenum.c
@@ -27,8 +27,8 @@
 #include <stdbool.h>
 #include <stdlib.h>
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 #include "qstr.h"
 #include "nlr.h"
 #include "obj.h"