- Add a missing space for styling
- Replace legacy "\`...\`" with `$(...)`
- Quote variable to prevent word splitting
- Use builtin `command -v` instead of non-standard `which`
- Add two missing `>&2` redirection for error/warning message

I've continually run into problems with the setup.cfg route that I'm not
interested in investing the time to debug and fix. Looking around it doesn't
seem to be very common yet so I've had a hard time finding help/docs etc.

define 3 kinds of requirements (base, dev, test)
retrieve version from __init__.py
define setuptools minimal version

in CI install full (base, dev, test) dependencies