Switch over to full GraphQL

[lilyminium]

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Fixes #370, I think.

This PR stops using PyGithub entirely and switches over to using GraphQL directly. It's not strictly necessary since the only issue with PyGithub was to do with projects, but as the project card objects are passed into every state-update function, I only realized this after rewriting the whole thing. The main overhead added is that we now use the GraphQL API to add/remove labels, get files, and add comments. If this is too much maintenance overhead I can try to smush together this PR and #375.

The new code is in lifecycle-v2.py. I added a test CI that runs through some steps (e.g. adding/removing labels, adding comments, moving cards around) that should only trigger on PRs labelled with test-ci. That's what's repsonsible for the mess here and in #375.