py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/unicode.c b/py/unicode.c
index c8faa57..88f8351 100644
--- a/py/unicode.c
+++ b/py/unicode.c
@@ -26,8 +26,8 @@
 
 #include <stdint.h>
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 
 // attribute flags
 #define FL_PRINT (0x01)