Bump pytest-cov from 2.8.1 to 2.9.0

Bumps [pytest-cov](https://github.com/pytest-dev/pytest-cov) from 2.8.1 to 2.9.0.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest-cov/compare/v2.8.1...v2.9.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

 -r requirements.txt
 attrs==19.3.0
 pytest==5.4.2
-pytest-cov==2.8.1
+pytest-cov==2.9.0
 mock==2.0.0
 flake8==3.8.2
 pyflakes==2.2.0