Bump Flake8 version to at least v3.9.2

[brynpickering]

Flake8 is now at version 6.0.0, I.e., 3 major releases ahead of the version Hound uses. I'm not proposing an update beyond Flake8 v3, but it would be great if you could bump the version up to the most recent version without backwards-incompatibilities, i.e. v3.9.2 (as per Flake8 release notes).

For one, this would fix the issue of Hound triggering F811 when using function overloading with type hinting (fixed in early 2021). I'm sure there would be other benefits to the update too!

I had a look at creating a PR directly, but cannot find any reference to the version of flake8 being set in this repository's source code...

[brynpickering]

To add to the benefits of updating to v3.9.2: Flake8 v3.6.0 triggers E231 and E225 when using valid assignment expressions (a.k.a. walrus operators).