build(deps): 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>

travis/requirements.txt

@@ -2,7 +2,7 @@ freezegun==0.3.12
 pytest==5.1.2
 pytest-asyncio==0.10.0
 responses==0.10.6
-pytest-cov==2.7.1
+pytest-cov==2.8.1
 pytest-random-order==1.0.4
 pytest-travis-fold==1.3.0
 codecov==2.0.15