black: add a pyproject.toml that contains our black settings
Differential Revision: https://phab.mercurial-scm.org/D6993
[tool.black]
line-length = 80
exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'