[CI] .coveragerc is ignored on CI

[pedro-psb]

After Dynconf began using the Codecov GitHub app this week, the coverage report is ignoring .coveragerc passing rules (fail under 95%) and is often failing with very small coverage changes.

This solution looks simple: use codecov.yml file. We just need to ensure a clean integration with both local and CI setups.

[pedro-psb]

For now, this has been set up correctly via GH Apps and .coveragerc should be kept for local reports.

It is not very good that we have two sources of coverage definitions (Apps and the file), but that's a more general issue and should be addressed when migrating to pyproject.toml #929, so I'll close this.