Switch to Python3.9 as default interpreter

Python3.9 has been available for a while on Toolforge,
and 3.7 (pinned in 9d5e608e) is not supported there anymore.

• Update base image in Dockerfile
• Update python_version in Pipfile
• Update tox target
• Add .python-version for pyenv usage

We confirmed the application works with the new version
in unit tests and docker-compose setup.

Pre-requisite for T319813.

Bug: T319813