Ignore "Interrupted system call" errors in PubSubWorkerThread

[vitawasalreadytaken]

The pubsub worker thread was sometimes dying on me silently because my process was interrupted by a signal while pubsub.get_message -> pubsub.parse_response -> connection.can_read -> select.select was blocking.

This fix simply ignores InterruptedErrors raised during the select poll and retries. I have been testing this in production for about half a year now.

Note that this is only needed for Python < 3.5. In 3.5 select ignores interruptions and retries automatically (https://docs.python.org/3/library/select.html#select.select).

[andymccurdy]

This is similar to #743 which was just merged. But #743 doesn't take into account select.

cc @carltongibson, thoughts?

[carltongibson]

My initial thought is to handle this in the connection (in can_read) where the select call occurs rather than the PubSubWorkerThread.

For neatness, it'd be nice to wrap it in a _compat.py function too.

But, yeah, I think we should handle this.

[andymccurdy]

I merged @carltongibson's change which should fix this.

[vitawasalreadytaken]

Cool, that's a nicer way to handle this. Thanks.