py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/modsys.c b/py/modsys.c
index 519d470..1e7f7ef 100644
--- a/py/modsys.c
+++ b/py/modsys.c
@@ -24,8 +24,8 @@
  * THE SOFTWARE.
  */
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 #include "qstr.h"
 #include "obj.h"
 #include "builtin.h"