Update pytest-doctestplus to 0.6.0

[pyup-bot]

This PR updates pytest-doctestplus from 0.5.0 to 0.6.0.

Changelog

0.6.0

==================

- Drop support for ``python`` versions earlier than 3.6. [103]

- Drop support for ``pytest`` versions earlier than 4.0. [103]

- Fix compatibility with ``pytest`` 5.4. [103]

Links

• PyPI: https://pypi.org/project/pytest-doctestplus
• Changelog: https://pyup.io/changelogs/pytest-doctestplus/
• Homepage: https://astropy.org

[pyup-bot]

Closing this in favor of #137