Merge branch '121-upgrade-to-python36' into 'develop'

Resolve "Random issue on import"

Closes #121

See merge request funkwhale/funkwhale!113

api/Dockerfile

-FROM python:3.5
+FROM python:3.6
 
 ENV PYTHONUNBUFFERED 1
 

api/docker/Dockerfile.test

-FROM python:3.5
+FROM python:3.6
 
 ENV PYTHONUNBUFFERED 1
 

changes/changelog.d/121.bugfix

+Upgraded to Python 3.6 to fix weird but harmless weakref error on django task (#121)