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.

setuptools v64.0.0 is used to support editable installs
via its PEP-660 implmentation
https://github.com/pypa/setuptools/pull/3488

This patch was taken nearly verbatim from the equivalent nova change.

Change-Id: I0f1f5af46ac6914bfddc847988d815825492949f
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

pyproject.toml

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

requirements.txt

@@ -2,7 +2,7 @@
 # date but we do not test them so no guarantee of having them all correct. If
 # you find any incorrect lower bounds, let us know or propose a fix.
 
-pbr>=3.1.1 # Apache-2.0
+pbr>=6.0.0 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 oslo.messaging>=9.4.0 # Apache-2.0
 Flask>=1.0.0 # BSD

setup.py

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