fetcher py2 fix: no deque.copy()

Author: dpkp

kafka/consumer/fetcher.py

@@ -613,7 +613,8 @@ def _fetchable_partitions(self):
         fetchable = self._subscriptions.fetchable_partitions()
         # do not fetch a partition if we have a pending fetch response to process
         # use copy.copy to avoid runtimeerror on mutation from different thread
-        discard = {fetch.topic_partition for fetch in self._completed_fetches.copy()}
+        # TODO: switch to deque.copy() with py3
+        discard = {fetch.topic_partition for fetch in copy.copy(self._completed_fetches)}
         current = self._next_partition_records
         if current:
             discard.add(current.topic_partition)