simplify: coveralls reporting

[Justintime50]

Description

The previous implementation of coveralls used a Python package to generate lcov reports because pytest-cov previously didn't support it; however, just a month ago support was added. This PR removes the unneeded coveralls dependency and uses the same flow as our other repos to report which is much simpler

NOTE: This won't work until we drop support for Python 3.6. Drafting till then.

Testing

Pull Request Type

Please select the option(s) that are relevant to this PR.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement (fixing a typo, updating readme, renaming a variable name, etc)