I've just upgraded (using homebrew), my git-review today from 1.26 to 1.27. I've got Python 3 installed on my system as well (macOS Sierra).
When I try to submit patch for review using git review -R, I get this error;
/usr/local/Cellar/git-review/1.27.0/libexec/vendor/lib/python3.7/site-packages/requests/__init__.py:91: RequestsDependencyWarning: urllib3 (1.24) or chardet (3.0.4) doesn't match a supported version! RequestsDependencyWarning)
I've searched online and came up empty, maybe this is a compatibility issue with the latest version of git-review on macOS Sierra with Python packages?
I fixed it with brew reinstall git-review, you can have a try.