py, unix: Make "mpconfig.h" be first included, as other headers depend on it.

Specifically, nlr.h does.
diff --git a/py/objset.c b/py/objset.c
index db3683f..1d10f94 100644
--- a/py/objset.c
+++ b/py/objset.c
@@ -2,9 +2,9 @@
 #include <string.h>
 #include <assert.h>
 
+#include "mpconfig.h"
 #include "nlr.h"
 #include "misc.h"
-#include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
 #include "runtime.h"