Libraries vs apps and pipenv

[greenape]

FlowKit is a mixture of libraries and applications. For the libraries we should probably not be pinning with pipenv and instead be using tox (or more modern equivalent) to test a matrix of dependencies.

Covers flowclient, flowmachine (in library mode - perhaps another reason to look at #1917), arguably flowkit-jwt-generator, and the flowetl python module (but not the image).