Get caching working in GH Actions build

[MJJoyce]

#471 will add a baseline GH Action workflow for testing the toolkit across all supported Python versions, running the docs build to verify that it completes successfully, and executing the linting pipeline.

The caching supported by actions/setup-python is currently disabled because it caused issues with the lint environment in the Python 3.10 tox execution. For some reason it consistently made pre-commit unavailable. Should test this more thoroughly and see if we can figure out a workaround. The caching really helps speed stuff up so it would be great to address this.