Add back Circle CI for publishing code coverage

[tqtg]

Description

Previously, we had Circle CI workflow to publish code coverage to codecov.io which shows the badge in our README. It stopped working at some point, we then removed it together with updating GitHub actions. Let's try to add it back for tracking test coverage. Alternatively, this could be added as part of the GitHub actions.

Related Issues

Checklist:

• I have added tests.
• I have updated the documentation accordingly.
• I have updated README.md (if you are adding a new model).
• I have updated examples/README.md (if you are adding a new example).
• I have updated datasets/README.md (if you are adding a new dataset).

[darrylong]

@tqtg Was thinking if we should utilize Github Actions instead so we don't have to maintain both CircleCI and Github Actions.
Or is there a feature we would like to use in CircleCI that Actions doesn't have?

[tqtg]

not really about any particular feature that we want from CircleCI, just that we can use a separate CI tailored for checking additional information (e.g., code coverage, code lint). GitHub actions can be designated for building and checking compatibility across different Python versions.

[darrylong]

@tqtg That sounds fine to me. While I noticed that there's an updated circleci python orb, shall we first get the current version working first before bumping the version up in another PR?