black: rename pyproject.toml to black.toml Black won't read this automatically (you'll have to specify --config), but having a pyproject.toml *at all* puts pip in PEP 517/518 mode which breaks us for obscure reasons I don't understand. Rather than waste a ton of time fighting with pip, let's just do this. Differential Revision: https://phab.mercurial-scm.org/D7087

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