suport upgrading pbr setup.py/setup.cfg to native setuptools declarative

[graingert]

https://docs.openstack.org/pbr/3.0.0/#setup-cfg

[graingert]

there's some differences, eg packages automatically has setuptools.find_packages behaviour where setuptools setup.cfg does not

[asottile]

I don't use / don't know anything about pbr so someone else would have to do this