Considerably simplify html_util for Python 3.10 compatibility (#58)
    
    As reported in #58 and RHBZ #1972391, `formatter` was removed
    from the Python standard library in Python 3.10. This heavily
    simplifies `html_util.html_to_text()` by using the stdlib
    `HTMLParser` class, which avoids the use of `formatter`.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>