Pytest-flake8dir

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* Update Python support to 3.5-3.8, as 3.4 has reached its end of life.
* Converted setuptools metadata to configuration file. This meant removing the
``__version__`` attribute from the package. If you want to inspect the
installed version, use
``importlib.metadata.version("pytest-flake8dir")``
(`docs <https://docs.python.org/3.8/library/importlib.metadata.html#distribution-versions>`__ /
`backport <https://pypi.org/project/importlib-metadata/>`__).

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* Run ``flake8`` in a ``subprocess`` rather than trying to get a speed boost by
running it in the current process. This is to overcome plugin state not
resetting in-process in ``flake8`` 3.7.0+.

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* Drop Python 2 support, only Python 3.4+ is supported now.

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* A temporary ``setup.cfg`` file is now always created with no options and
passed as ``--config``, to avoid flake8 merging in user-specific settings.
Use ``make_setup_cfg`` to set the contents of this file.

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* The exit code from ``flake8`` is now saved on the ``Flake8Result`` object.
Any tests that relied on catching ``SystemExit`` themselves will need
refactoring to use the new attribute for their assertions.

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* Add convenience methods ``make_example_py`` and ``make_file``.