I can't reproduce this failure locally on Arch Linux - same versions of Python and pip. Try restarting the build?

It works for me locally too and I tried multiple times. I’ve even reverted it on master after multiple attempts. :-/

On Twitter, @gaborbernat suggested this could be due to pypa/setuptools#1506. Does seem like the same general problem, but I'm not sure why you'd be picking up an old version of setuptools in the isolated build environment, and I still can't reproduce the problem even if I explicitly pin setuptools to an old version in the build-system.requires, which is extra puzzling.

I've found Travis's Python images sometimes have moderately outdated package versions bundled in.

Maybe mypy is out of date...what happens if you add a pip install -U --force-reinstall mypy to your install: section of .travis.yml?