repopick: support auth'ing to gerrit and picking drafts

* Use requests if installed. If not fall back to urllib. This is
  done because users may or may not have requests installed and
  requiring them to do so for simple http stuff isn't really
  reasonable.

* If requests is installed and a .gerritrc file exists in the
  user's HOME directory, try to get credentials out of it
  for the given gerrit instance. If auth for the correct gerrit
  instance exists, use it to auth to gerrit. If no .gerritrc
  exists, just use requests with no auth.

  Example ~/.gerritrc entry:
  review.lineageos.org|invisiblek|httppasswordhere

Change-Id: I95be26d51bfd31b53f3613e8dbfb7bba46324571