I'd like to mention that using setup_requires breaks pip with Python 3.5 on macOS because setup_requires dependencies are installed by easy_install, which uses the system OpenSSL, which only supports TLS 1.1 on macOS + Python 3.5. But PyPI only allows TLS 1.2.

See urllib3/urllib3#1587 (comment) for more details if you're interested. I don't think this requires an action on your part, but I thought it would be useful to record this here.