issue33217 will not be "fixed" with a TypeError, but incorrect Falses are also bad.

Rather than add __contains__ to IntFlag (and every other Flag mixin), I think the best answer is to adjust Flag.__contains__ a little bit more to check if `other` is of the same type as the mixin class, and if so see if it's one of the values.

Thank you for being thorough and finding this problem as well.