Add verification check for minimum screenpy dependency

[bandophahita]

It occurred to me as I was reviewing a PR that we don't currently have a check to ensure screenpy_selenium works with the minimum screenpy version supported.

Example: we consolidated namespaces in screenpy so nearly everything could be imported directly from the root.
If screenpy_selenium supported a version of screenpy prior to that consolidation but proceeded to import everything as if it assumed they could be found at the root, users would get import errors.

I'm thinking we could create a specific tox environment (?) where we specifically install the oldest supported screenpy and run tests/linters/etc.