py: Add "io" module.

So far just includes "open" function, which should be supplied by a port.

TODO: Make the module #ifdef'ed.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 368b0fc..06548d0 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -98,6 +98,7 @@
 Q(getattr)
 Q(hash)
 Q(id)
+Q(io)
 Q(int)
 Q(isinstance)
 Q(issubclass)