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

Specifically, nlr.h does.
diff --git a/py/opmethods.c b/py/opmethods.c
index a745ec1..b029eef 100644
--- a/py/opmethods.c
+++ b/py/opmethods.c
@@ -1,6 +1,6 @@
+#include "mpconfig.h"
 #include "nlr.h"
 #include "misc.h"
-#include "mpconfig.h"
 #include "qstr.h"
 #include "obj.h"
 #include "runtime.h"