Fix appveyor builds

[sambhav]

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

[sambhav]

See discussion on tox-dev/tox#791

Basically tox is using a really old version of pip which doesn't support some newer features required by our deps.

[sambhav]

@Nurdok all the appveyor builds are failing. Please merge this so the other PRs pass.

[Nurdok]

Thanks!