make: run flake8 on specific directories

[SnoopJ]

Description

Tin. Run the linter on only the files we are concerned with. This is slightly friendlier to developers who have throwaway scripts or a virtual environment (other than one in env/) in their repository directory.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make lint and make test)
• I have tested the functionality of the things this change touches

[dgw]

Looked a couple of times, didn't find any more declarative (in a config file somewhere) way to do this. The current state also bit me earlier this week, though, so let's accept this Good solution in lieu of a hypothetical Perfect one yet to be found.