Update pluggy to 0.4.0

[pyup-bot]

There's a new version of pluggy available.
You are currently using 0.3.1. I have updated it to 0.4.0

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

Changelog

0.4.0

---

• add has_plugin(name) method to pluginmanager. thanks nicoddemus_.

• fix 11: make plugin parsing more resilient against exceptions
  from __getattr__ functions. Thanks nicoddemus.

• fix issue 4_: specific HookCallError exception for when a hook call
  provides not enough arguments.

• better error message when loading setuptools entrypoints fails
  due to a VersionConflict. Thanks blueyed_.

.. _11: pytest-dev/pytest#11
.. _4: pytest-dev/pytest#4

.. _blueyed: https://github.com/blueyed
.. _nicoddemus: https://github.com/nicoddemus

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 #104