Deprecate and remove `pipenv`

[ml-evs]

Now that we have essentially removed pipenv, we should plan to deprecate the residual Pipfile and Pipfile.lock that remain for convenience. This should be done at the next-next minor version (v0.6.0) so people have time to migrate any hacky setups they have with pipenv right now. It's a bit hard to formally deprecate it, but perhaps an intermediate case would be to move the files somewhere new (whilst retaining them in the repo) before deleting in v0.6.0 along with the documentation sections.