Load-balanced xdist

I wrote a pytest plugin to evenly balance tests across xdist workers.Back story: the coverage.py test suite seemed to be running oddly: it would run to near-completion, and then stall before actually finishing. To understand why, I added some debug output to see what tests were running on which workers.I have some very slow tests (they create a virtualenv and install packages). It turned out those tests were being run near the end of the test suite, after their worker had already run a bunch of ...