Pathlib2 was used to support Python 3.5, but we're not using that version
any more anyway.

Haven't written the mypy-running unit test yet, that'll come when all
warnings are solved. This commit doesn't solve all of them, just some
simple ones.

This also means that we need a `tests/__init__.py` file; this file makes
py.test undertand what is our sources directory, and thus allows them to
`import flamenco_worker`. As a result, the test imports from tests/*.py
need to change to relative imports.