Add coverage report and integration with codecov

[morcuended]

This will help us to identify parts of the code not tested

[morcuended]

Reviving this issue.

Do we want to depend on third-party services such as codecov, sonarqube or simply upload the generated coverage report (https://hynek.me/articles/ditch-codecov-python/). I would go for the latter.

[juanep97]

I would also go for the latter and avoid third-party services.