I'm peeking a look in the code trying to understand the impliciation
and if this is the best fix.

Am I right that an alternative solution would be to implement __eq__
and keep the old-style class?

Because this probably would be the only new style class in that area
and I wonder about consistency. If we wanted to go to new style classes,
why not make HTMLInputMixin a newstyle class? What about the other
objects. in cgi/templating.py