using_ci() reports whether a continuous integration
environment is being used.
using_appveyor() reports whether AppVeyor is being used
using_circle_ci() reports whether CircleCI is being used
using_codebuild() reports whether AWS CodeBuild is being used
using_github_actions() reports whether GitHub Actions is being
used
using_gitlab_ci() reports whether GitLab CI/CD is being used
using_jenkins() reports whether Jenkins is being used
using_travis_ci() reports whether Travis CI is being used