Use new pip resolver

[anders-kiaer]

As for now, this is a check of the new pip resolver. We will merge this when the new resolver actually becomes the default (in October, and with the release of pip==20.3).

[anders-kiaer]

pip==20.3 has passed planned release date by 18 days, still not released. Suggest we add this, and also add a information box in readme stating that people using pip < 20.3 (which anyway would be quite some also after new pip release should enable the new resolver).

The new resolver makes it possible to merge #318 (pip < 20.3 gives dependency version conflicts on that one).

[HansKallekleiv]

👍