Bump pytest from 5.4.3 to 6.0.0 (#139)

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](pytest-dev/pytest@5.4.3...6.0.0)

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