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.

This is required to support installing devstack on
centos stream 9 and related distros with GLOBAL_VENV=True
Without this change the wsgi scripts are not generated in
editable mode. i.e. pip install -e /opt/stack/keystone

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

setuptools v64.0.0 is used to support editable installs
via its PEP-660 implmentation
pypa/setuptools#3488

Change-Id: Ie31840df3c78013f49def3c5b95c8f79589ad830

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["pbr>=5.7.0", "setuptools>=64.0.0", "wheel"]
+build-backend = "pbr.build"