Update pluggy to 0.13.1

[pyup-bot]

This PR updates pluggy from 0.8.1 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+.

0.12.0

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

Features
--------

- `215 <https://github.com/pytest-dev/pluggy/issues/215>`_: Switch from ``pkg_resources`` to ``importlib-metadata`` for entrypoint detection for improved performance and import time.  This time with ``.egg`` support.

0.11.0

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

Bug Fixes
---------

- `205 <https://github.com/pytest-dev/pluggy/issues/205>`_: Revert changes made in 0.10.0 release breaking ``.egg`` installs.

0.10.0

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

Features
--------

- `199 <https://github.com/pytest-dev/pluggy/issues/199>`_: Switch from ``pkg_resources`` to ``importlib-metadata`` for entrypoint detection for improved performance and import time.

0.9.0

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

Features
--------

- `189 <https://github.com/pytest-dev/pluggy/issues/189>`_: ``PluginManager.load_setuptools_entrypoints`` now accepts a ``name`` parameter that when given will
load only entry points with that name.

``PluginManager.load_setuptools_entrypoints`` also now returns the number of plugins loaded by the
call, as opposed to the number of all plugins loaded by all calls to this method.



Bug Fixes
---------

- `187 <https://github.com/pytest-dev/pluggy/issues/187>`_: Fix internal ``varnames`` function for PyPy3.

Links

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

[coveralls]

Pull Request Test Coverage Report for Build 248

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.222%

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Totals
Change from base Build 146:	0.0%
Covered Lines:	35
Relevant Lines:	36

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💛 - Coveralls

[pyup-bot]

Closing this in favor of #109