Comment 4 for bug 1998086

Same problem again...

Could not determine the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If you want to investigate this yourself the log files in '/var/log/dist-upgrade' will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.

---
I did of course run as instructed:
~> sudo ppa-purge ppa:deadsnakes/ppa
Updating packages lists
PPA to be removed: deadsnakes ppa
Warning: Could not find package list for PPA: deadsnakes ppa
~> sudo apt update
...
~> sudo apt upgrade
...