I think you have three options:
-
leave things as they are, so continue to run 20.10 (now EOL)
Remove problematic PPA by
sudo add-apt-repository -r ppa:deadsnakes/ppa
then retry package list update with
sudo apt-get update
Then use local old python versions by https://github.com/pyenv/pyenv-installer .
-
backup current system and then upgrade 20.10 to supported 21.04 using standard desktop procedure
sudo apt-get update sudo apt-get dist-upgrade sudo reboot sudo do-release-upgrade
Notes:
- I have tested this method on my RPi 4B without any issues.
- deadsnakes PPA does not provide packages for 21.04.
-
backup current system and do a clean install of 20.04 LTS (focal) which is supported by deadsnakes PPA. Then enable USB boot on it.