More unit tests?

Is there interest in adding more unit tests to pymavlink? I've been attempting to use for some Python3 projects, but without much success.

Looking at the ``.travis.yml`` file, it appears that the unit testing is fairly light-on.

I've previously added a suite of unit tests to CanberraUAV's imaging project (https://github.com/CanberraUAV/cuav), so I thought I could do something similar here.

Start by reviewing .travis.yml to understand the existing test setup and how Python 3 projects are exercised. Inspect the current unit-test coverage and identify a specific, agreed testing gap before making changes; done should include a focused test suite that runs successfully in the project’s CI configuration.

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