Bump pytest-cov from 2.5.1 to 2.8.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.5.1 to 2.8.1.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest-cov/compare/v2.5.1...v2.8.1)

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

requirements-dev.txt

@@ -3,4 +3,4 @@ ddt==1.2.0
 flake8==3.5.0
 mock==3.0.5
 pytest==3.6.3
-pytest-cov==2.5.1
+pytest-cov==2.8.1