Running mitmproxy on macOS Sierra: traffic not passing through

Question

I installed mitmproxy with homebrew.

mitmproxy --version
Mitmproxy version: 2.0.0 (release version)
Python version: 3.6.0
Platform: Darwin-16.3.0-x86_64-i386-64bit
SSL version: OpenSSL 1.1.0e  16 Feb 2017
Mac version: 10.12.2 ('', '', '') x86_64

mitmproxy -T --host

And then specify the browser in the mitm.it file, and I see the message "If you see this, traffic does not go through mitmproxy." Is there a known issue or fix? If not, where should I research?