Upgrading python froom 3.8 to 3.9 on mint 20:

sudo apt-get install python3.9
pip3 freeze --path /home/wuff/.local/lib/python3.8/site-packages/ > python3.8_requirements.txt 
pip3 install -r python3.8_requirements.txt
sudo pip install --upgrade $(pip list --outdated --format freeze | cut --fields=1 --delimiter="=")

pip list --outdated --format freeze | cut --fields=1 --delimiter="=" > python3.9_upgrade.txt 
pip3 install -r python3.9_upgrade.txt 

error from beets relating to gstreamer not finding python-gi:

cd /usr/lib/python3/dist-packages/gi
sudo ln -s _gi.cpython-{38,39}-x86_64-linux-gnu.so

removal of old python versions afterwards on mint 20.3:

sudo apt-get purge python2
sudo apt-get purge libpython2
sudo apt-get purge libpython2-stdlib 
sudo apt-get purge python2.7
sudo apt-get purge python2.7-minimal 
sudo apt-get purge libpython2.7
sudo apt-get purge libpython2.7-minimal 
sudo apt-get purge python2-minimal 
sudo apt-get purge python3.6
sudo apt-get purge python3.6-minimal
sudo apt-get purge libpython3.6-minimal