Bump pytest from 5.3.5 to 6.0.2

Bumps [pytest](https://github.com/pytest-dev/pytest) from 5.3.5 to 6.0.2.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@5.3.5...6.0.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements_dev.txt

@@ -7,7 +7,7 @@ flask==1.1.1
 mock==4.0.1
 pip==20.0.2
 plumbum==1.6.8
-pytest==5.3.5
+pytest==6.0.2
 PyYAML==5.3
 Sphinx==2.4.1
 tox==3.14.3