listing.py: pylint cleanups (including a refactor)
    
    This is a bit bigger than the other pylint cleanups; it includes
    refactoring _get_tcms to split out the detailed parsing for the
    three broad types of pages we handle into three separate methods,
    which greatly improves the readability.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>