return bytes (not str) from fetch_xml()

return bytes (not str) from fetch_xml()

broke pull-subs logging when it choked on an unrelated error
Traceback (most recent call last):
  File "/opt/yt/app/common/utils.py", line 151, in <module>
    pull_subscriptions(verbosity, force, limit)
  File "/opt/yt/app/common/utils.py", line 44, in pull_subscriptions
    pull_feed(feed_id, feed_type, conn, verbose)
  File "/opt/yt/app/common/utils.py", line 67, in pull_feed
    f.write(xmlfeed + b"\n")
TypeError: must be str, not bytes