Log Message:
Updated py-test-runner to 2.10.1.

2.10.1
~~~~~~

* #21: Avoid mutating dictionary keys during iteration.

2.10
~~~~

* #20: Leverage technique in `setuptools 794
  <https://github.com/pypa/setuptools/issues/794>`_
  to populate PYTHONPATH during test runs such that
  Python subprocesses will have a dependency context
  comparable to the test runner.