Update pyflakes to 2.4.0

[pyup-bot]

This PR updates pyflakes from 2.3.1 to 2.4.0.

Changelog

2.4.0

- Remove unused tracing code (``traceTree``)
- Add support for ``match`` statement
- Detect ``typing`` module attributes when imported with ``import ... as ...``

Links

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

[pyup-bot]

Closing this in favor of #519