Bump pytest-cov from 2.7.1 to 2.8.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.7.1 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](pytest-dev/pytest-cov@v2.7.1...v2.8.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/tests.txt

@@ -5,6 +5,6 @@ pbr==5.1.3
 pluggy==0.12.0
 py==1.8.0
 pytest==4.4.1
-pytest-cov==2.7.1
+pytest-cov==2.8.1
 six==1.12.0
 python-coveralls==2.9.2