Make it more obvious which Python version runs on a given Linux distribution (#15909)

Make it more obvious which Python version runs on a given Linux distribution so when we end up dropping support for a given Python version, we can more easily find the reference to the Python version and remove any references for the distribution. We don't want to be running tests or building packages on a distribution that no longer has a supported Python version.

This way, we can avoid another situation like when we dropped support for Python 3.7 but forgot to drop the Debian Buster references everywhere (https://github.com/matrix-org/synapse/pull/15893)