Possibility of using locked semantic versioned dependencies?

[IanTayler]

Currently sppl depends on hard-coded strict-equality versions of its dependencies.

This makes its inclusion to ongoing projects somewhat problematic.

We always have the choice of vendoring the code instead of installing it through pip, and ignoring restrictions on the setup.py, but I think it'd be best if sppl used a system similar to poetry or pip-tools, with loosely-restricted versions for use as a library, and strict, locked versions for experiment reproducibility in the papers, etc.

Is this something you have considered?

[femtomc]

@fsaad I can vouch for poetry and show you how to setup this project to use it.