Merge branch 'renovate/docker-python-3.x' into 'master'

Update python Docker tag to v3.9

See merge request infrablue/infrablue!37

Dockerfile

-FROM python:3.8-buster AS core
+FROM python:3.9-buster AS core
 
 # Configure Python to be nice inside Docker and pip to stfu
 ENV PYTHONUNBUFFERED 1