Use PBR as a PEP 517 build backend

Recent versions of PBR are now capable of being integrated as a
build backend per the PEP 517 specification, so that the setup.py
script no longer needs to be invoked directly by anything. Declaring
setup_requires is now done via a new pyproject.toml file instead.
This allows us to avoid some recent deprecations in SetupTools,
though the file itself needs to be retained as a stub for now since
it still gets imported by PBR's wrapper of the SetupTools build_meta
hooks.