The instructions @ericklan posted are on the right track, though they aren’t quite clear to me and seem to recommend more than is necessary. I pieced together the answer from the above and a helpful post in the bug thread.

These are the steps I took to correct this:

Back up your my.cnf file in /etc/mysql and remove it
Remove the folder /etc/mysql/mysql.conf.d/
Verify you don’t have a my.cnf file stashed somewhere else (I did in my home dir!) or in /etc/alternatives/my.cnf.
Backup and remove /etc/mysql/debian.cnf files (not sure if needed, but just in case)
sudo apt purge mysql-server mysql-server-5.7 mysql-server-core-5.7
sudo apt install mysql-server
In case your syslog shows an error like “mysqld: Can’t read dir of ‘/etc/mysql/conf.d/'” create a symbolic link ln -s /etc/mysql/mysql.conf.d /etc/mysql/conf.d Then the service should be able to start with service mysql start.
That got it working!
mysql

16.04 upgrade broke mysql-server
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