I don't know too much about tox, but i installed latest python (3.7.2) on my local dev machine (windows) and tried to run your build script and ran into the same error as on the build VMs.
Are there any other special steps i should take to make it build locally?
Not reallly. Can you confirm you can run `tox -e py36` or `tox -e py35` but not `tox -e py37`?
I am assuming you do have Python 3.7 available in your system, and that it is installed under C:\\Python37-x64, as it (should be?) installed in AppVeyor. At least that is what I have configured in the `.appveyor.yml` file, and used to work fine up until now..
I did some more testing locally and was able to make this command run if i skipped production of a virtualenv.
Again, I don't have much experience with python development (especially tox testing) but it seems that tox creates its own virtualenv and so there is some confusion between which python.exe to use?
Here is a build of your repo with the same approach which succeeds.
Well as i demonstrated earlier, it's not an AppVeyor-only problem since it's on my local machine as well.
Feel free to ask me any questions about my local machine, if you'd like to figure out the differences between it and your local machine.
On travis, you're building on linux I assume?... not really comparable.
What do you make of skipping virtualenv production leading to a successful build? Does tox not create its own virtualenv?