I think there may be a way to accomplish this in a reasonably straightforward fashion in python3 given that feedparser has an 'unreadline' function.  The python2 case is probably going to be a more complicated change.  And I agree that multiple lines should be examined.

There is also the question of what to do with the extra data.  I think the correct approach is to treat a line that does not conform as a line that is missing the leading whitespace that would make it a continuation line.  It looks like that would certainly be correct for the case in hand.