abandon_stale.py: send data as a dict, not a string

After a recent PR merged on pygerrit2 [1], sending data as a string
no longer works as expected; it must be sent as a dict.

[1] https://github.com/dpursehouse/pygerrit2/pull/10

Change-Id: Idaaff88e9811bf8ca90ef2719f13022ffd09cfad

contrib/abandon_stale.py

@@ -191,7 +191,7 @@ def _main():
 
         try:
             gerrit.post("/changes/" + change_id + "/abandon",
-                        data='{"message" : "%s"}' % abandon_message)
+                        data={"message" : "%s" % abandon_message})
             abandoned += 1
         except Exception as e:
             errors += 1