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.

Change-Id: Id7156e4069cce7ffbdda6ab5cf1af9c5a4d23b67
Signed-off-by: Dmitriy Rabotyagov <noonedeadpunk@gmail.com>

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["pbr>=6.0.0", "setuptools>=64.0.0"]
+build-backend = "pbr.build"