fix: Deal with backward incompatible changes in bleach.

The `bleach` library now expects sets as input for a few parameters and
their defaults have been updated to reflect that.  So we needed to do
some set unions instead of adding lists together in a few places.

Details of the changes can be found here: https://bleach.readthedocs.io/en/latest/changes.html#version-6-0-0-january-23rd-2023