Use pytest, get rid of test/runner.py

[giampaolo]

See #2446.

This PR:

• gets rid of psutil/tests/runner.py (less code to maintain)
• use pytest-xdist to allow for parallel test execution
• get rid of concurrencytest dep
• update Github and Appveyor CI config
• removes 400 lines of code

Replacing self.assert* APIs will be done in a separate PR.