Replace Heroku with something else (GitHub Actions?)

[pllim]

Reasons:

• Heroku + GitHub API been flaky lately.
• Heroku logs hard to decipher.
• Heroku access is a bottleneck and extra maintenance burden.
• Updating upstream dependency on Heroku can be tricky because it is cached.

This issue supersedes #122 .

Replacements that are already deployed in astropy

• astropy_bot/autolabel.py ➡️ labeler (see astropy)
• baldrick/plugins/github_pull_requests_base_branch.py ➡️ TST: Add base branch check astropy#11158 using actions/github-script (POCs here, here, and here)
• baldrick/plugins/github_towncrier_changelog.py ➡️ actions-towncrier-changelog (BOT: Proof-of-concept using Action to check towncrier astropy#11489 is a stop-gap fix until @Cadair can figure something out as permanent fix)
• baldrick/scripts/stale_issues.py and baldrick/scripts/stale_pull_requests.py ➡️ MNT: Revive Astropy stalebot in dry-run mode (ported baldrick logic to Action) astropy#12131

Replacements that work but not deployed

• baldrick/utils.py ➡️ action-special_pr_comment (POCs here, here, here, here, and here)
  • p.s. I did temporarily deploy it in astropy on "special day" when astropy-bot was broken but I commented it back out after.

Replacements that need upstream fixes

• baldrick/plugins/github_milestones.py ➡️ See the github-script example in action-check_milestone_exists (blocked by feature-request-add-milestone-changes-as-activity-type-to-pull-request)

No longer relevant

• astropy_bot/changelog_checker.py ➡️ action-check_astropy_changelog (POCs here and here) -- This has been made obsolete by Use a YAML file as the basis for the change log astropy#10334

[pllim]

I took a stab at replacing the milestone check with custom GitHub Action but it is blocked by https://github.community/t/feature-request-add-milestone-changes-as-activity-type-to-pull-request/16778/7 .

From that exercise, my conclusions are as follow: It will be pretty much a complete refactoring moving from astropy-bot to Actions. A lot of the things that make sense now won't make sense in Actions. For example, we don't need TOML config because you can customize the Actions itself. We can use PyGithub instead of custom GitHub API, probably no need for Flask app too, etc etc.

Also not sure how to write unit tests for a custom GitHub Action.

UPDATE: Going the GitHub Action route, implementing it in TypeScript would be much more efficient. Using Python + Docker has at least a 20s overhead and that is using a custom Docker image stored at quay.io with dependencies installed.

[pllim]

Priorities...