Document three ways to get the dependencies since Debian's system pip is locked
down (PEP 668 externally-managed-environment): pip, apt (no pip), or a venv, with
the note that `python3 -m lintunes.main` runs the app without pip or the
`lintunes` command at all. Make the second-machine steps install-agnostic and add
the app screenshot.
trav
