Use setuptools 5.0 (==3.8.1) because of a regression in 4.0.1

See https://rbcommons.com/s/twitter/r/517/ and
https://mail.python.org/pipermail/distutils-sig/2014-June/024478.html

Testing Done:
CI passed

Reviewed at https://rbcommons.com/s/twitter/r/520/