Add pyproject.toml

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Python has been generating related to out-of-date use of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.