Use Poetry as package dependency manager

[rod-glover]

We want a tool to:

• manage package dependencies: abstract and concrete (locking)
• manage virtual environments
• natively support installation and execution in multiple versions of Python

After some research (Pipenv, Poetry, PDM, Flit), Poetry is currently likely the best fit for our needs.

PDM does this too, but looks less mature and has features we don't need. That said, we may decide to transition to it at some future point, and that should not be too onerous. They do essentially address the same things, in slightly different ways.