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] <[email protected]>

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -9,7 +9,7 @@ docker==4.1.0
 flake8==3.7.8
 ipdb==0.12.2
 motor==2.0.0
-pytest-cov==2.7.1
+pytest-cov==2.8.1
 pytest-sugar==0.9.2
 pytest==5.2.1
 python-dateutil==2.8.0