Add pyproject.toml

pip 23.1 removed the "setup.py install" fallback for projects that do
not have pyproject.toml. This change adds the minimal pyproject.toml
to enable pbr to be properly used to build editable wheels.

[1] https://docs.openstack.org/pbr/latest/user/using.html#pyproject-toml

Change-Id: Ie14fb4db0da3b20daadfd55647c8563e65a338d2
Closes-Bug: #2109645

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["pbr>=6.1.1"]
+build-backend = "pbr.build"

setup.py

@@ -13,9 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# THIS FILE IS MANAGED BY THE GLOBAL REQUIREMENTS REPO - DO NOT EDIT
 import setuptools
 
-setuptools.setup(
-    setup_requires=['pbr>=2.0.0'],
-    pbr=True)
+setuptools.setup(pbr=True)