Github Actions: Python tox tests are failing on MacOS

[kylophone]

I merged #774/#775 to get our tox tests in CI again. This was necessary to do right away, since the tox tests need to be run in at least some form otherwise no other PRs could safely be merged. There is still an issue however, when tox is run on MacOS there are some Python issues which are not present on Linux. The failures seem to have more to do with Python environment and/or dependencies than actual code correctness. Obviously, we should try to fix these ASAP.

Example failure: https://github.com/Netflix/vmaf/runs/1579841877#step:10