Replace travis CI/AppVeyor with CircleCI

[badboy]

Note: This is currently a very reduced feature set.

• Only stable
• Only Linux

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Note that while this removes the old config, the services are still set up to do work. Trying to figure out who can help with that.

[saschanaz]

I'd still prefer GitHub Actions unless there's a good reason to prefer CircleCI, 1) just because I'm more familiar with GHA 2) that way we won't need to go another website with another permission to modify settings 👀

(We should also fix README badges btw)

[badboy]

I'm ok with GhA. We don't need anything special, so all already-allowed actions should be good enough for us. I would leave the setup to you then though.