Bump attrs from 19.1.0 to 19.3.0

Bumps [attrs](https://github.com/python-attrs/attrs) from 19.1.0 to 19.3.0.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/python-attrs/attrs/compare/19.1.0...19.3.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-tests.txt

 -r requirements.txt
-attrs==19.1.0
+attrs==19.3.0
 pytest==5.4.2
 pytest-cov==2.8.1
 mock==2.0.0