Set a new minimum Python version

[kjd]

Review downstream dependencies and identify a new reasonable minimum Python version, with a goal to preserving compatibility but removing increasing technical debt associated with older versions.

[CAM-Gerlach]

NB, 3.8 is the earliest version we support (security-only) upstream, but if you're considering distros, 3.7 might be a reasonable minimum as that's what RHEL 8 is stuck on.

Also, it can't fix everything, but pyupgrade (or its Ruff equivalents, minus the six support) is a useful tool for removing and modernizing cruft for older Python versions.