pyproject: add config file

This will tell pip et al to call our setup.py for the majority of
packaging concerns, but also gives us a place to put standard config
stuff like black.

Differential Revision: https://phab.mercurial-scm.org/D9833

pyproject.toml

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

tests/test-check-code.t

@@ -70,6 +70,7 b' Prevent adding new files in the root dir'
   hg
   hgeditor
   hgweb.cgi
+  pyproject.toml
   rustfmt.toml
   setup.py