openff.interchange should be a dependency

[Clownshift]

Hi Devs,

I know the project is in an early stage, but I would already like to try it out. I installed it from conda-forge with micromamba, which seemed to have worked pretty well and also quickly.
However, when I tried to run the example from the user-guide

I received an error when initializing the runner:
prepared_system_a, prepared_system_b = absolv.runner.setup(system, config, force_field)
failed, because openff.interchange could not be imported.

I believe openff.interchange should be included in the dependencies, right?

[SimonBoothroyd]

Good catch! I think it probably should be...