mygpo: Send timestamp as UTC to gpodder.net (bug 1036)

Although time.time() returns UTC times,datetime.datetime.fromtimestamp()
then translates that to local time. *utc*fromtimestamp() should be used.

src/gpodder/my.py

@@ -408,7 +408,7 @@ def synchronize_episodes(self, actions):
         log('Starting episode status sync.', sender=self)
 
         def convert_to_api(action):
-            dt = datetime.datetime.fromtimestamp(action.timestamp)
+            dt = datetime.datetime.utcfromtimestamp(action.timestamp)
             since = mygpoutil.datetime_to_iso8601(dt)
             return api.EpisodeAction(action.podcast_url, \
                     action.episode_url, action.action, \