update-md-links doesn't work with mistune v3

Version 3 of the mistune Python library came out last June, and it's what pip installs by default. But when I use it with update-md-links, I get an error, since the API is not what the script expects. I have downgraded to version 2.

To avoid this trouble for the next person, I would suggest:

• Possibly, update update-md-links to use mistune version 3.
• In any case, have update-md-links check the expected version of mistune, and tell the user what to install if it isn't present.

cc @Diziet