Set setuptools min in requirements.txt

If the version of setuptools is old enough that it does not support pyproject.toml but the version of pip is new enough that it does, pip will use pbr’s PEP-517 backend which will attempt to use setuptools’ which will fail as it doesn’t exist due to setuptools being too old.

Set a minimum version for setuptools in requirements.txt. We were already doing this in pyproject.toml but not here.