Fix flake8

flake8 refuses to start with pyflakes 2.2.x. Work around this, without downgrading pyflakes, by shipping the current master of flake8 and the unmerged patch that makes it compatible with 2.2.x. I've tried to apply the patch on top of the latest release, but that did not work. This version appears to be working fine however, and I think it's the best we can do in this situation. I'll add a warning to py3-pyflakes to prevent this in the future.

• Related: https://gitlab.com/pycqa/flake8/-/merge_requests/417
• Fixes: #11378 (closed)