Idea: make use of coveralls ability to merge coverage data of multiple builds

[abravalheri]

Describe your use-case

Currently we only measure coverage for a single specific Python version and OS combination. This means that a lot of parts of the code have a # pragma: no cover directive...

I recently noticed that coveralls is able to merge coverage info: https://docs.coveralls.io/parallel-build-webhook

Maybe we could leverage that in the future.

Describe the solution you would like to see for your use-case

• Cirrus CI setup (and others) should add a COVERALLS_PARALLEL=true environment variable
• The last step in the CI pipeline should call coveralls --finish