Declare test dependency on pytest using extras_require

[cottsay]

This same approach to declaring test dependencies has been taken in several ros-infrastructure and colcon packages, and allows tools like colcon to invoke the appropriate testing framework.

Fedora has also adopted this approach in packaging automation: https://docs.fedoraproject.org/en-US/packaging-guidelines/Python/#_test_dependencies_2

[cottsay]

Done. The pytest-cov dependency is a little hazy. It isn't strictly a requirement or a dependency to invoke the tests, which is why I omitted it from the list in setup.py. Let me know if you think I should just list it there to reduce duplication.