CI Builds are failing due to editable install

[pganssle]

Currently the CI builds are all red because rustapi_module has a pyproject.toml, and something is invoking pip install -e (see error here).

This is related to pypa/pip#6434. I think pinning pip to an older version (I think pip<19.0) should temporarily fix the issue while we work out what to do. I'm not entirely sure why anything is doing an editable install, though.

[gaborbernat]

https://github.com/PyO3/pyo3/blob/master/tox.ini#L11 sets develop mode one, which in practice means editable install 👍