Convert `setup.py` to `pyproject.toml` which is the modern way of
defining a Python project.  This change avoids various warnings that
Python has been generating related to out-of-date use of
`setup.py`.

Other notable changes:

* The `scap` entrypoint is now
  prepared automatically when the package is installed due to this
  part of `pyproject.toml`:
```
[project.scripts]
scap = "scap:cli.Application.run"
```

* Renamed `bin/scap` to `bin/run-dev-scap`.  The latter is only used
  during documentation generation.  Update the references in
  `docs/scap2/commands.rst` and `docs/scap3/deploy_commands.rst`.

* Deal with a change in behavior in `black` when `pyproject.toml` is
  present.  It found a couple of trailing commas that it wanted to
  add in `scap/checks.py` and `scap/cli.py`.

Change-Id: I9312d60945432eeccd8e8406e60607cb66768576