add at least a nominal pyproject.toml in root of repository

[d-w-moore]

The nominal form (spelled out under the link below) should at least make sure we're covered.

See this on the official Python web site.

Note on issue #774 , which isn't quite the same. That one refers to a comprehensive conversion, into a real pyproject.toml, of dependencies and specifications currently stored in setup.py

[alanking]

Nominal pyproject.toml file was added in #772. Closing.