feat(dev): enable gitpod development

[miketheman]

Use the initalization phase to install all of the Python versions
and run the tests on startup, making subsequent test runs faster.

Signed-off-by: Mike Fiedler miketheman@gmail.com

[miketheman]

I don't think we have GitPod enabled on the pypa org yet, but this could be a starter on a simpler repo to demonstrate the desired behaviors.

Test this out with a URL like: https://gitpod.io/#https://github.com/pypa/readme_renderer/pull/238 (use your own personal GitHub auth).

Without Prebuilds enabled, it'll take some time to run all the init commands, but eventually completes and runs tox to prove a "clean slate" before the developer makes any changes.

We could also try to create our own custom Dockerfile instead, but that seemed like a heavier lift, since we'd have to push the image to DockerHub or another registry, etc.

Looking forward to your thoughts!