black: add a pyproject.toml that contains our black settings

Differential Revision: https://phab.mercurial-scm.org/D6993

pyproject.toml

@@ -0,0 +1,3 b''
+[tool.black]
+line-length = 80
+exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'

tests/test-check-code.t

@@ -66,6 +66,7 b' Prevent adding new files in the root dir'
   hg
   hgeditor
   hgweb.cgi
+  pyproject.toml
   setup.py
 
 Prevent adding modules which could be shadowed by ancient .so/.dylib.