Bump pytest from 5.3.5 to 6.0.1

Bumps [pytest](https://github.com/pytest-dev/pytest) from 5.3.5 to 6.0.1.
- [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/5.3.5...6.0.1)