Run tests with coverage reporting

[akx]

Fixes #775.

[akx]

Things seem to work: https://app.codecov.io/github/eclipse/paho.mqtt.python/pull/782/tree

@PierreF Would you like me to see if we can get the coverage% as a GitHub comment without an external service?

If we want Codecov to comment on the PRs, someone admin enough on eclipse will need to enable this repo on https://app.codecov.io/ .

[PierreF]

I think codecov could be fine, it seems already used by few other project from Eclipse.

@akx did you know which "admin enough" right are needed ? @icraggs do you know who to ask for (if not you).

[akx]

@PierreF Presumably someone who can see eclipse on https://github.com/apps/codecov/installations/select_target :)

[icraggs]

Ok, I think I've done everything needed. Let me know if not.

[PierreF]

@akx I've re-run one of the tox job. Should we expect something on this PR or should I merge it and we should see it on future PR ?

[akx]

@PierreF I think hopefully we'll see Codecov comment in a subsequent PR after this gets merged :) Thanks!