Use pbr

pbr is the standard build system for packages maintained under
the OpenStack namespace. Although setup.py has been indicated that
pbr is required pyproject.toml hasn't agreed with it.

Change-Id: I756698cc1a3608843c8bfb2fd9adf6b2f5d07e47
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

Author: kajinamit

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
-requires = ['setuptools>=68']
-build-backend = 'setuptools.build_meta'
+requires = ["pbr>=6.1.1"]
+build-backend = "pbr.build"
 
 [project]
 name = "python-observabilityclient"