Support github actions CI

[ArturKlauser]

This adds support for CI running on github actions.

Getting the branch name is a little more tricky since the default way of repository cloning with actions/checkout puts you in a detached HEAD in which the regular git rev-parse --abbrev-ref HEAD returns HEAD instead of the branch name.

[TheKevJames]

This changeset looks good, thanks for the contribution! I'll set up Github Actions on this repo for testing and, assuming all goes well, release this in the next version bump.

[ArturKlauser]

Seems like this is running into the weirdest issue with the coveralls API. The new coveralls-python 1.9.x works all peachy under Github Actions until it comes to the point of submitting the data to coveralls.io. Its API seems to steadfastly refuse a service name of github. It appears to accept pretty much anything else. It's totally weird.

coveralls-python$ GITHUB_ACTIONS=true GITHUB_REF=refs/heads/master GITHUB_HEAD_REF='' COVERALLS_REPO_TOKEN=mytoken \
coveralls --service github
Submitting coverage to coveralls.io...
Could not submit coverage: 422 Client Error: Unprocessable Entity for url: https://coveralls.io/api/v1/jobs
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/coveralls/api.py", line 185, in wear
    response.raise_for_status()
  File "/usr/local/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 422 Client Error: Unprocessable Entity for url: https://coveralls.io/api/v1/jobs

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/coveralls/cli.py", line 77, in main
    result = coverallz.wear()
  File "/usr/local/lib/python3.7/site-packages/coveralls/api.py", line 188, in wear
    raise CoverallsException('Could not submit coverage: {}'.format(e))
coveralls.exception.CoverallsException: Could not submit coverage: 422 Client Error: Unprocessable Entity for url: https://coveralls.io/api/v1/jobs

coveralls-python$ GITHUB_ACTIONS=true GITHUB_REF=refs/heads/master GITHUB_HEAD_REF='' COVERALLS_REPO_TOKEN=mytoken \
coveralls --service github-ci
Submitting coverage to coveralls.io...
Coverage submitted!
Job ##19.1
https://coveralls.io/jobs/56404049

When you compare coveralls dump it's identical apart from the service name. I also tried service names of github-actions, github-actions-ci, anything. They all work, just service name github is rejected by the coveralls API server.

I suggest we rename the service to github-actions. I'll send you another pull request with that later.

[TheKevJames]

Well that's... odd. Alrighty, github-actions makes sense to me.