Restore the CI environment

Restore the CI environment

This is a bulk change that tries to bring up the CI environment back
to health. It does:

- switch to newer Python versions (3.7+ only)
- switch to current codecov action
- drops no-longer-supported codecov-results-as-artifact storage
- install dependencies for, and build documentation (as validation
  step during the build, not to use the result)

Hopefully this will allow again CI to run correctly.