Update pytest-cov to 2.12.1

[pyup-bot]

This PR updates pytest-cov from 2.12.0 to 2.12.1.

Changelog

2.12.1

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

* Changed the `toml` requirement to be always be directly required (instead of being required through a coverage extra).
This fixes issues with pip-compile (`pip-tools1300 <https://github.com/jazzband/pip-tools/issues/1300>`_).
Contributed by Sorin Sbarnea in `472 <https://github.com/pytest-dev/pytest-cov/pull/472>`_.
* Documented ``show_contexts``.
Contributed by Brian Rutledge in `473 <https://github.com/pytest-dev/pytest-cov/pull/473>`_.

Links

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

[pyup-bot]

Closing this in favor of #1704