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.txt

@@ -1,7 +1,7 @@
 async-generator==1.10; python_version>="3.6"
 codacy-coverage==1.3.11
 mock==3.0.5
-pytest-cov==2.7.1
+pytest-cov==2.8.1
 pytest-tornasync; python_version >= '3.5'
 pytest-trio==0.5.2; python_version>="3.6"
 pytest==4.5.0