Non-docker upgrade to 0.20.0 breaks Funkwhale

Hi people!

I've upgraded my non-docker version of Funkwhale to the version 0.20.0, and the process was not that smooth.

What happened

The upgrade fails to install uvicorn and breaks a funkwhale service. The result is a 502 bad gateway message after the upgrade if you try to access your funkwhale instance.

What I've learned

To upgrade to Funkwhale 0.20.0 it is necessary to install uvicorn, but uvicorn 0.8.6 requires websockets 8, which in turn requires python >=3.6.

Debian stretch uses python 3.5 and an upgrade to debian buster is necessary to solve this problem.

After the system is upgraded, the python virtual environment used by funkwhale has to be deleted and rebuilt again. (The step Python dependencies on the installation manual has to be redone.)

My suggestions

So I would like to suggest an update to the upgrade manual to reflect the hard dependency on python 3.6. It would be good to instruct people before the upgrade process to check their python versions.

Many Thanks! :)