Update pytest-cov to 2.10.0

[pyup-bot]

This PR updates pytest-cov from 2.9.0 to 2.10.0.

Changelog

2.10.0

-------------------

* Improved the ``--no-cov`` warning. Now it's only shown if ``--no-cov`` is present before ``--cov``.
* Removed legacy pytest support. Changed ``setup.py`` so that ``pytest>=4.6`` is required.

Links

• PyPI: https://pypi.org/project/pytest-cov
• Changelog: https://pyup.io/changelogs/pytest-cov/
• Repo: https://github.com/pytest-dev/pytest-cov