Update pluggy to 0.13.1

[pyup-bot]

This PR updates pluggy from 0.12.0 to 0.13.1.

Changelog

0.13.1

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

Trivial/Internal Changes
------------------------

- `236 <https://github.com/pytest-dev/pluggy/pull/236>`_: Improved documentation, especially with regard to references.

0.13.0

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

Trivial/Internal Changes
------------------------

- `222 <https://github.com/pytest-dev/pluggy/issues/222>`_: Replace ``importlib_metadata`` backport with ``importlib.metadata`` from the
standard library on Python 3.8+.

Links

• PyPI: https://pypi.org/project/pluggy
• Changelog: https://pyup.io/changelogs/pluggy/
• Repo: https://github.com/pytest-dev/pluggy

[pyup-bot]

Closing this in favor of #442