pip 19.1 prevents pytest from being used in editable mode

[nicoddemus]

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

[gaborbernat]

pypa/pip#6434 issue report and discussion, things should be better once pip 19.1.1 gets out (hopefully soon)

[bskinn]

For (2), perhaps try removing -e . (or pip install -e ., or whatever mechanism) for the pip-mediated editable install, and instead running python setup.py develop to achieve an editable install via setuptools.

This has worked fine so far for both of my PEP517 projects.