build(deps): bump pytest-cov from 2.6.0 to 2.8.1

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.6.0 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@v2.6.0...v2.8.1)

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

Author: dependabot-preview[bot]

test-requirements.txt

@@ -3,6 +3,6 @@ pytest-forked==1.1.0
 tox==3.7.0
 Werkzeug==0.15.5
 pytest-localserver==0.4.1
-pytest-cov==2.6.0
+pytest-cov==2.8.1
 gevent
 eventlet