add pyproject.toml to support pip 23.1

pip 23.1 removed the "setup.py install" fallback for projects
that do not have pyproject.toml and now uses a pyproject.toml
which is vendored in pip.
To address that, this change adds the minimal pyproject.toml
to enable pbr to be properly used to build editable wheels.

See https://pip.pypa.io/en/stable/news/#v23-1
and https://github.com/pypa/pip/issues/8368 for more
details on the removal of the fallback support.

Co-Authored-By: Sean Mooney <work@seanmooney.info>
Change-Id: I16b04ea0aa1ecda809dc4f614a91cdc70a25d881

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["pbr>=6.1.1"]
+build-backend = "pbr.build"