Bleach is deprecated, so switch to nh3

[jiri-one]

Since bleach is already obsolete/deprecated and will not evolve further, it would be better to switch to nh3, which is not only actively evolving, but also significantly faster than bleach.

Relevant links about bleach and also about nh3:
nh3 docs - https://nh3.readthedocs.io/en/latest/
nh3 github - https://github.com/messense/nh3
discussion and announcement of the end of bleach - mozilla/bleach#698

[parisk]

Thanks for your contribution @jiri-one. We cannot merge this currently, as it's an important decision we need to make calmly. We do have plenty of time, since the maintainers will continue to push security fixes for the foreseeable future, but we certainly need to make a decision here.

Closing this PR for now and switching this over to a discussion (#38), so we can lay down and examine all options.

[jiri-one]

@parisk NP, it is up to you :). this code will exist in the future ... ;).