Add pyproject.toml

This blocks #6264 because black configuration is stored in pyproject.toml.
See <https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#configuration-format>
I copied this from <https://setuptools.pypa.io/en/latest/userguide/quickstart.html#basic-use>

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"