Bump pytest-cov from 2.5.1 to 2.6.0

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.5.1 to 2.6.0.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.5.1...v2.6.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

requirements-dev.txt

@@ -1,5 +1,5 @@
 flake8==3.6.0
 pytest==3.5.0
 pytest-asyncio==0.9.0
-pytest-cov==2.5.1
+pytest-cov==2.6.0
 pudb==2018.1