pyproject.toml

formatting: introduce a `test-check-format-black.t` that enforce formatting

This should prevent use to drift away from the expect format.

[tool.black]
line-length = 80
exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'