Add devel/py-pytest-cov, pytest plugin for measuring coverage

This plugin produces coverage reports. It supports centralised testing and
distributed testing in both load and each modes. It also supports coverage of
subprocesses.

All features offered by the coverage package should be available, either through
pytest-cov or through coverage's config file.

WWW: https://github.com/pytest-dev/pytest-cov

Reviewed by: koobs
Differential Revision: D12394