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Mistune has moved several functions from scanner to util, see this commit.
So you have to change from mistune.scanner import escape_html, escape_url to from mistune.util import escape_html, escape_url in /opt/mailman/mm/venv/lib/python3.9/site-packages/hyperkitty/lib/renderer.py as the error message suggests.
PS: Use pip freeze | grep -i django, because it appears as Django.
PS2: Seems like other mailman3 users are as unhappy as you are about the mistune dependency, see their gitlab bugtracker. This also means that hyperkitty devs should be aware of the new stable mistune version (they had a dependency on the 2.0 release candidate after which the refactoring in question happened).
Source:stackexchange.com