Improve CI Resource Usage

[blink1073]

Follows what was done to address pypa/setuptools#3093

[pradyunsg]

Concurrenty

Thanks for filing this PR! Could you please fix this typo in the commit message: concurrency?

[blink1073]

Thanks for filing this PR! Could you please fix this typo in the commit message: concurrency?

Haha thanks, fixed

[takluyver]

Thanks! I see this is part of what @di suggested in his email to pypa-committers. While we're here, should we also disable builds on pushes to branches other than main, i.e.:

on:
  push:
    branches:
      - main
  pull_request:

(When I make a PR myself, I'll probably push the branch to this repo, because there's a redirect from takluyver/flit to pypa/flit so long as I don't fork it)

[blink1073]

Sounds good, will follow up when I get back to keyboard

[takluyver]

Thanks @blink1073 !