Use pytest instead of unittest

[giampaolo]

I recently did this for pyftpdlib https://github.com/giampaolo/pyftpdlib/pull/628.

Reasons:

• use bare assert statements as opposed to unittest's self.assert* APIs (easier to read, shorter to type)
• get rid of psutil/tests/runner.py (less code to maintain) - DONE in #2453
• pytest-xdist extension allows for parallel test execution, so we can get rid of our internal implementation (less code to maintain) - DONE in #2447
  • as such get rid of concurrencytest dep
• in production environments, where pytest may not be installed, we still want to be able to test psutil installation via python3 -m psutil.tests by using unittest test runner - DONE in #2456.

[giampaolo]

Fixed in https://github.com/giampaolo/psutil/pull/2447, https://github.com/giampaolo/psutil/pull/2453, #2456, #2459, #2461.