Add pytest-xdist and print test durations at the end
Add pytest-xdist and print test durations at the end

Parallelizing the tests takes the run time from ~30s to ~15 locally.
Further, printing the test durations should help us identify the tests
to target to improve the test time. While the test runs aren't terribly
slow, they could definitely stand to improve.

sigmavirus24/github3.py