diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,3 @@
+[tool.black]
+line-length = 80
+exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'
diff --git a/tests/test-check-code.t b/tests/test-check-code.t
--- a/tests/test-check-code.t
+++ b/tests/test-check-code.t
@@ -66,6 +66,7 @@
   hg
   hgeditor
   hgweb.cgi
+  pyproject.toml
   setup.py
 
 Prevent adding modules which could be shadowed by ancient .so/.dylib.