py: Include mpconfig.h before all other includes.

It defines types used by all other headers.

Fixes #691.
diff --git a/py/qstr.c b/py/qstr.c
index 7ccb34f..10bc20e 100644
--- a/py/qstr.c
+++ b/py/qstr.c
@@ -27,8 +27,8 @@
 #include <assert.h>
 #include <string.h>
 
-#include "misc.h"
 #include "mpconfig.h"
+#include "misc.h"
 #include "qstr.h"
 
 // NOTE: we are using linear arrays to store and search for qstr's (unique strings, interned strings)