Set up caching for GitHub Actions (pip, mypy)

Examples:

- https://github.com/pallets/werkzeug/blob/main/.github/workflows/tests.yaml#L41-L42
- https://github.com/pallets/werkzeug/blob/main/.github/workflows/tests.yaml#L48-L53

Ref:

- https://github.com/actions/setup-python/pull/266