Fix Python 3.4 error in circus-top. Fix #891

We were deleting elements of a dictionary when iterating over it
via `filter`. As `filter` returns an iterator in Python 3.4, this
was causing a "dictionary changed size during iteration" error.

circus/stats/client.py

@@ -250,9 +250,7 @@ def main():
                     subtopic = 'all'
 
                 # Clean pids that have not been updated recently
-                def valid_pid(p): return p.isdigit() and p in watchers[watcher]
-
-                for pid in filter(valid_pid, watchers[watcher]):
+                for pid in tuple(p for p in watchers[watcher] if p.isdigit()):
                     if (last_refresh_for_pid[pid] <
                             time.time() - int(args.process_timeout)):
                         del watchers[watcher][pid]