pip 19.1 prevents pytest to use editable mode (pypa/pip#6370) due to our pyproject.toml file.

This is a problem for us for two reasons:

1. tox.ini uses usedevelop in a release and docs environments. I think we can get away from using it in docs, not so sure about release.

2. We use editable mode ourselves.

I will open a PR shortly trying to address 1, but for 2 I don't know of any workaround other than downgrading pip manually.

cc @RonnyPfannschmidt