This doesn't work because PIP_VERSION is consumed by PIP. Need to rename the env variable.

I understand the gist of what this is trying to solve, but I think this is better solved with tox and environment matrixes.

I think we should close this out and reimplement it after the PR switching to tox has landed.

Regardless, I'm taking this out of the 1.0 milestone.

It would be nice to test the minimum versions of dependencies that we support. Luckily, there really only is six and webencodings that are required. six will go away when Python 2 support is gone.

However, we should NOT be pinning maximum versions as overly restrictive version caps cannot be fixed by the user. See: https://iscinumpy.dev/post/bound-version-constraints/

In any case, this PR is quite outdated. I'll make a new one to declare minimum versions of all dependencies that we still use in master.