add pyproject.toml to support pip 23.1

pip 23.1 removed the "setup.py install" for projects that do not have
their own pyproject.toml by having a hardcoded one within 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 pypa/pip#8368 for more info.

Change-Id: I7f922a97db4bfab374172db7db3e60db9d0b7940
Signed-off-by: Doug Goldstein <doug.goldstein@rackspace.com>

Author: cardoe

pyproject.toml

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

setup.py

@@ -16,5 +16,5 @@
 import setuptools
 
 setuptools.setup(
-    setup_requires=['pbr>=2.0.0'],
+    setup_requires=['pbr>=6.0.0'],
     pbr=True)