The version of dev-python/pbr in our tree is from late 2018 (v5.1.1). Gentoo is at 5.8.0 and upstream at 5.8.1. Currently our version of dev-python/pbr fails to emerge with python-3.7
This package should be autogennable.
Note: I discovered this while doing a `vuNDa world` emerge which is remerging a bunch of packages for changes python USE flags