don't include unittests in wheel

[BubuOT]

Hi, I'm currently packaging varlink in buildroot and noticed that the package unittests are included in the wheel (and thus in the installed package in python3.x/site-packages/varlink/). While the unittests are potentially useful at packaging time and thus it's nice to include them in the sdist, I don't really want them to be present in my final python installation as they are just dead code there. (Varlink currently heavily references the tests from the readme as an installed module, which makes them part of the package's public API in a way... so maybe that should be changes as well for that reason.)