Add minimum pyproject.toml

... which is not required by the requirements-check job.

Change-Id: I94d3a3e961284e3981ca0fe244e443ae944fd353
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

Author: kajinamit

pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
-requires = ["setuptools", "pbr"]
-build-backend = "setuptools.build_meta"
+requires = ["pbr>=6.0.0", "setuptools>=64"]
+build-backend = "pbr.build"