- 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