Update pluggy to 0.5.1

[pyup-bot]

There's a new version of pluggy available.
You are currently using 0.5.0. I have updated it to 0.5.1

These links might come in handy: PyPI | Changelog | Repo

Changelog

0.5.1

---

• fix a bug and add tests for case where firstresult hooks return
  None results. Thanks to RonnyPfannschmidt_ and tgoodlet
  for the issue (68) and PR (69) respectively.

.. _69: pytest-dev/pytest#69
.. _68: https://github.com/pytest-dev/pytest/issuses/68

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖