Bump pytest from 5.4.3 to 6.0.0

Bumps [pytest](https://github.com/pytest-dev/pytest) from 5.4.3 to 6.0.0.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/5.4.3...6.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

 -r requirements.txt
 attrs==19.3.0
-pytest==5.4.3
+pytest==6.0.0
 pytest-cov==2.10.0
 mock==2.0.0
 flake8==3.8.3