Bump pytest-cov from 2.8.1 to 2.9.0 (#270)

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](pytest-dev/pytest-cov@v2.8.1...v2.9.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

tests/requirements.txt

@@ -6,7 +6,7 @@ pycodestyle==2.5.0
 pyflakes==2.2.0
 pytest==5.4.2
 pytest-aiohttp==0.3.0
-pytest-cov==2.8.1
+pytest-cov==2.9.0
 pytest-isort==1.0.0
 pytest-mock==3.1.0
 pytest-sugar==0.9.3