Update pluggy to 0.5.1

[pyup-bot]

There's a new version of pluggy available.
You are currently using 0.4.0. 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

0.5.0

---

• fix bug where callbacks for historic hooks would not be called for
  already registered plugins. Thanks vodik_ for the PR
  and hpk42_ for further fixes.

• fix 17_ by considering only actual functions for hooks
  this removes the ability to register arbitrary callable objects
  which at first glance is a reasonable simplification,
  thanks RonnyPfannschmidt_ for report and pr.

• fix 19: allow registering hookspecs from instances. The PR from
  tgoodlet also modernized the varnames implementation.

• resolve 32: split up the test set into multiple modules.
  Thanks to RonnyPfannschmidt for the PR and tgoodlet_ for
  the initial request.

• resolve 14: add full sphinx docs. Thanks to tgoodlet for
  PR 39_.

• resolve 44: move to new-style classes. Thanks to MichalTHEDUDE
  for PR 46_.

• add baseline benchmarking/speed tests using pytest-benchmark
  in PR 54. Thanks to tgoodlet.

• update the README to showcase the API. Thanks to tgoodlet_ for the
  issue and PR 55_.

• deprecate __multicall__ and add a faster call loop implementation.
  Thanks to tgoodlet_ for PR 58_.

• raise a comprehensible error when a hookimpl is called with positional
  args. Thanks to RonnyPfannschmidt_ for the issue and tgoodlet_ for
  PR 60_.

• fix the firstresult test making it more complete
  and remove a duplicate of that test. Thanks to tgoodlet_
  for PR 62_.

.. _62: pytest-dev/pytest#62
.. _60: pytest-dev/pytest#60
.. _58: pytest-dev/pytest#58
.. _55: pytest-dev/pytest#55
.. _54: pytest-dev/pytest#54
.. _46: pytest-dev/pytest#46
.. _44: pytest-dev/pytest#44
.. _39: pytest-dev/pytest#39
.. _32: pytest-dev/pytest#32
.. _19: pytest-dev/pytest#19
.. _17: pytest-dev/pytest#17
.. _14: pytest-dev/pytest#14

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

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #49