Migrate to Pipenv
Pipenv guarantees deterministic builds via Pipenv.lock
which are important (IMO) when we're mixing versioned packages and git packages and leaving a messy environment behind.
We would need to remove documentation related to running elixi.re with system python or running it behind a manually-managed virtualenv, which (also IMO) is good, Pipenv can manage a virtualenv better than manual work could.
This is just a suggestion (as per tags) so I ask for responses from @slcx and @ao