- consolidate at `prepare_python_and_pip()`
- sanity check for Python version 3.9+ regardless of HOSTRELEASE
- TODO: pip vs sudo/root: need pip 22.2+ to curb warning, not doing it
- completely remove support for building under `buster` -- that's way too old, sorry.
- de-hardcode `python3` invocations, instead use `python3_binary_path` set by `prepare_python3_binary_for_python_tools()`
- juggle `$HOSTRELEASE`: read from actual host, or determined from Docker image name (during Dockerfile build)
- TL;DR: include and use `python3.9` for focal-like host OS's