Update pytest-cov to 2.6.0

[pyup-bot]

This PR updates pytest-cov from 2.5.1 to 2.6.0.

Changelog

2.6.0

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

* Dropped support for Python < 3.4, Pytest < 3.5 and Coverage < 4.4.
* Fixed some documentation formatting. Contributed by Jean Jordaan and Julian.
* Added an example with ``addopts`` in documentation. Contributed by Samuel Giffard in
`195 <https://github.com/pytest-dev/pytest-cov/pull/195>`_.
* Fixed ``TypeError: 'NoneType' object is not iterable`` in certain xdist configurations. Contributed by Jeremy Bowman in
`213 <https://github.com/pytest-dev/pytest-cov/pull/213>`_.
* Added a ``no_cover`` marker and fixture. Fixes
`78 <https://github.com/pytest-dev/pytest-cov/issues/78>`_.
* Fixed broken ``no_cover`` check when running doctests. Contributed by Terence Honles in
`200 <https://github.com/pytest-dev/pytest-cov/pull/200>`_.
* Fixed various issues with path normalization in reports (when combining coverage data from parallel mode). Fixes
`130 <https://github.com/pytest-dev/pytest-cov/issues/161>`_.
Contributed by Ryan Hiebert & Ionel Cristian Mărieș in
`178 <https://github.com/pytest-dev/pytest-cov/pull/178>`_.
* Report generation failures don't raise exceptions anymore. A warning will be logged instead. Fixes
`161 <https://github.com/pytest-dev/pytest-cov/issues/161>`_.
* Fixed multiprocessing issue on Windows (empty env vars are not passed). Fixes
`165 <https://github.com/pytest-dev/pytest-cov/issues/165>`_.

Links

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