Address test coverage issues, add tests

This fixes some issues with showing coverage with tests run under multiprocessing. The fix is to add relevant coverage configuration under pyproject.toml.

In addition, I noticed that the test coverage includes the actual test modules as well, so this now omits those test modules (and correspondingly lowers the test coverage).

I also moved the entrypoint tests (e.g. kafka2frame --help) into the test suite as we can't append coverage that was run under multiprocessing. It also has the nice feature that we can run a single pytest command to run all the tests. One downside is that I need to skip these for python 3.6 as it lost support for the particular pytest plugin that is used to run these.

Finally, added a few tests:

• Run kafka2frame/frame2kafka under confluent-kafka as well.
• Add write_frame tests.

Closes #21.