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 [1][2]. pip 24.2 has now deprecated a similar fallback to
"setup.py develop" and plans to fully remove this in pip 25.0 [3][4][5].
pbr supports editable installs since 6.0.0 so we can get ahead of this
by adding our own minimal pyproject.toml to ensure we are using the
correct build system.

[1] https://pip.pypa.io/en/stable/news/#v23-1
[2] pypa/pip#8368
[3] https://pip.pypa.io/en/stable/news/#v24-2
[4] pypa/pip#11457
[5] https://ichard26.github.io/blog/2024/08/whats-new-in-pip-24.2/

Change-Id: I66b075b15a83e8d30803e92c36683aff569f3d7b
(cherry picked from commit 3e0b163)

Author: anfimovir

pyproject.toml

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

requirements.txt

@@ -3,7 +3,7 @@
 # you find any incorrect lower bounds, let us know or propose a fix.
 
 alembic>=1.8.0
-pbr!=2.1.0,>=2.0.0 # Apache-2.0
+pbr>=6.0.0 # Apache-2.0
 eventlet>=0.27.0 # MIT
 SQLAlchemy>=1.4.0 # MIT
 keystonemiddleware>=4.17.0 # Apache-2.0

setup.py

@@ -15,6 +15,4 @@
 
 import setuptools
 
-setuptools.setup(
-    setup_requires=['pbr>=2.0.0'],
-    pbr=True)
+setuptools.setup(pbr=True)