Filter sources passed to the pytest invocation.

[benjyw]

Problem

We were passing all sources from the entire dep closure to the pytest EPR. This included, for example, the requirements.txt file. So modifying requirements.txt, even by just adding a comment, would cause pytest to be re-run, even if the change had no effect on the actual set of requirements.

Solution

Only pass python sources and resources to pytest.
Only pass python sources to the coverage config.

Result

Whitespace changes to requirements.txt no longer invalidate the pytest EPR.