TST,BLD: require pytest-cov, latest versions of pytest,pytest-cov,pyest-runner

[westurner]

This adds a dev requirement for pytest-cov (coverage is already required); which adds support for the pytest --cov= --cov-report= options.

I also added the verbose pytest flag to the Makefile because the test output is so much more helpful; that's something I could obviously opinionatedly add to derivatives of this excellent cookiecutter.

In checking only the pytest-related package versions on pypi against those in requirements_dev:

• I noticed that there are newer versions and just changed all to current. Hopefully there's no version conflict.
• I noticed that pytest-runner now says pytest-runner is bad and wrong and just use tox. I basically always just run make test and never python setup.py pytest anyway, so.
  https://pypi.org/project/pytest-runner/