PyPI deploy failing on Windows: "Couldn't install pip, setuptools, twine or wheel."

I’ve noticed the last couple releases I’ve tried to make for some packages that use the “pypi” provider deployment are failing on Windows (and Windows only) with:

failed to deploy
-z was unexpected at this time.
Couldn't install pip, setuptools, twine or wheel.

This looks like it must be a flag that the deployment tools are adding. An example failing build: https://travis-ci.org/pytroll/pycoast/jobs/621782819

Any ideas what’s going on?

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