Fix #957: added mention of python 3.6 requirement (and workaround)

docs/admin/upgrading.rst

@@ -161,6 +161,13 @@ The following example assume your setup match :ref:`frontend-setup`.
 Upgrading the API
 ^^^^^^^^^^^^^^^^^
 
+.. note::
+
+    Starting with Funkwhale 0.20, it is recommended you run Funkwhale with Python 3.6 or greater. This means upgrading to
+    Debian 10 if you're on Debian 9.
+
+    If you still want to use Python 3.5, you can, but you'll need to run ``sudo -u funkwhale -H -E /srv/funkwhale/virtualenv/bin/pip install "uvicorn==0.8.6"`` before upgrading.
+
 On non-docker, upgrade involves a few more commands. We assume your setup
 match what is described in :doc:`/installation/debian`: