Add pyproject.toml to support pip version 23.1

Version 23.1 of pip removes the "setup.py install" if no
pyproject.toml is present.

This change adds a minimal pyproject.toml to use pbr to build
wheels.

See https://pip.pypa.io/en/stable/news/#v23-1
and pypa/pip#8368 for more info.

Author: elfosardo

pyproject.toml

@@ -0,0 +1,4 @@
+[build-system]
+requires = ["pbr>=6.0.0", "setuptools>=64.0.0"]
+build-backend = "pbr.build"
+