python 3.7 venv broken after upgrade to Ubuntu 20.04

Same problem for me. This is my solution if you do not want to upgrade everything (perhaps not all package are upgradable).

1. Install python 3.7 which is gone with upgrade to ubuntu 20

   sudo add-apt-repository ppa:deadsnakes/ppa
   sudo apt-get update
   sudo apt-get install python3.7
   

2. in your virtualenv dir (e.g env/) edit last line in pyenv.cfg

   version = 3.7
   

3. set back soft link of python3 in env/bin linking back to 3.7

   ln -s  /usr/bin/python3.7 python3
   

You may need to delete old symlik before creating new one

Now, should work: it does for me!

In my case, it was solved just by deleting and recreating the virtual env, and reinstalling Django, of course. After that, just reloaded Apache and everything worked again.