Run tox tests in parallel

[rmartin16]

Changes

• The tox -e py command now runs pytest tests in parallel
• Use tox -e py -- to run tests sequentially in process

Notes

• This took way longer to get working than I'd like to say...
• There are....some complications with getting coverage.py working with distributed tests
  • Not least of which is using an ostensibly bizarre pypi package that was uploaded 7 years ago..
  • I tried to get pytest-cov to work....but I couldn't stop some innocuous errors....and it seems possible from Ned's documentation he doesn't have much love for it.
  • At any rate, one of those two packages (or some black magic) is necessary to get Python loaded correctly for coverage.py to work

I was about to abandon this effort but I did finally get it working. That said, I'm not sure this is the form it should necessarily take. A competing PR will be created shortly.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct