Update flake8 to 4.0.1

[pyup-bot]

This PR updates flake8 from 3.9.0 to 4.0.1.

Changelog

3.9.2

-------------------

You can view the `3.9.2 milestone`_ on GitHub for more details.

Bugs Fixed
~~~~~~~~~~

- Fix error message for ``E111`` in ``pycodestyle`` (See also :pull:`1328`,
:issue:`1327`).

Deprecations
~~~~~~~~~~~~

- ``indent_size_str`` is deprecated, use ``str(indent_size)`` instead (See
also :pull:`1328`, :issue:`1327`).


.. all links
.. _3.9.2 milestone:
 https://github.com/PyCQA/flake8/milestone/40

3.9.1

-------------------

You can view the `3.9.1 milestone`_ on GitHub for more details.

Bugs Fixed
~~~~~~~~~~

- Fix codes being ignored by plugins utilizing ``extend_default_ignore`` (See
also :pull:`1317`)


.. all links
.. _3.9.1 milestone:
 https://github.com/PyCQA/flake8/milestone/38

Links

• PyPI: https://pypi.org/project/flake8
• Changelog: https://pyup.io/changelogs/flake8/
• Repo: https://github.com/pycqa/flake8

[pyup-bot]

Closing this in favor of #165