Migrate to Github Actions from Travis CI

[wren]

CC: #1007

Feature Request

We need to switch away from Travis CI (it's still getting worse). As mentioned in #1007, Circle seems like a good candidate.

We should start by mirroring our current setup, and running Travis and Circle side-by-side as a test.

Steps needed to complete this migration:

• linux tests
• mac tests
• windows tests
• linting
• accessibility testing for docs site (jrnl.sh)
• changelog generator
• if beta
  • send to pypi as prerelease
  • update jrnl prerelease tap
• if release
  • send to pypi
  • send updated pr to homebrew
• trigger a pipeline to do everything for a release
  • update version in correct files
  • tag repo

[wren]

Circle has nice tools, but we've been having some troubles with their billing. They give a bunch of free credits to open source projects which are only good for linux CI. I thought that would be fine, and we could just use those free linux credits, and pay for some extra mac and windows credits, but Circle doesn't like that. The moment you pay a single dollar for any service, they take away the open source free credits, so we'd also have to pay for linux builds. At their rates, it's prohibitive to pay for the linux, mac and windows entirely, so Circle doesn't seem like it's going to work out.

Github actions luckily doesn't have these restrictions and is completely free for open source repos. So, it's looking like that will likely be the direction we go in.

[wren]

I put homebrew automation in the checklist, but we already have a separate issue for that (#686), so I'll that part for that issue.