Use latest setuptools practice

[jamesmyatt]

In particular:

• Change script into entry point (https://setuptools.pypa.io/en/latest/userguide/entry_point.html#console-scripts) (like pytype: https://github.com/google/pytype/blob/main/setup.cfg).
• Avoid deprecated setup.py commands (https://setuptools.pypa.io/en/latest/deprecated/commands.html#running-setuptools-commands)

Note that you can also run the tool via python -m importlab too now.

Fixes #51

[jamesmyatt]

While this does fix #51 and enable you to use the CLI, e.g. via pipx. I'm not sure that the tools works on Windows, since most of the unit tests fail, but that's a separate issue:

...\tests>python -m unittest     
FF.FFFF...........F.FFF.......EEEEEEEEEEEEEEEEEEEE.EE...F..EEEE...E.EEEE..E.F.F.F....FE.
...
FAILED (failures=17, errors=35)