Add build information (PEP 517)

pyproject.toml

+[build-system]
+requires = ["setuptools >= 40.6.0", "wheel"]
+build-backend = "setuptools.build_meta"
+
 # See <https://pypi.org/project/black/>.
 
 # black defaults to "line-length = 88"; we prefer "line-length = 79" for now to