bleach is deprecated

[dgw]

As I hinted when converting the plugin to a proper package (b9b8d03), the bleach library is deprecated.

It would appear that the community has gravitated to nh3, Python bindings around a Rust library. Pros for that option include that nh3's API is pretty much a drop-in replacement for bleach.

There is also html-sanitizer, which offers element pre- and postprocessing, but it's unclear where to find documentation of the library other than reading its code. (Postprocessing could be useful for another thought I had about this plugin: Converting limited HTML tags to equivalent IRC formatting. Depends on how it works, though!)