CI unpin flake8, only run flake8-rst in pre-commit

[MarcoGorelli]

• closes CI: Linting errors from flake >= 3.8.1 #34150 (maybe?)
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[simonjayhawkins]

you can probably remove the pip install of pyflakes>=2.2.0 now.

[MarcoGorelli]

Turns out this can be simplified - no need for a fork/mirror, we can just pin flake8-rst and flake8 in additional_dependencies and run it as a local hook

[fangchenli]

The new pip dependency resolver will become default soon https://discuss.python.org/t/announcement-pip-20-2-release/4863. I tested it earlier. It got stuck if flake8 is pinned using < sign.

Also

The conflict is caused by:
    The user requested pyflakes>=2.2.0
    flake8 3.7.9 depends on pyflakes<2.2.0 and >=2.1.0

So we probibly need to merge this before pip 20.3 release.

[jreback]

can you merge master and ping on green.

Is there a way to have an option in code_checks.sh that actually runs the appropriate pre-commit hook? that way we have a single source of truth, and you can actually still run them (of course have to avoid running these on the CI from the script as well).

[MarcoGorelli]

Sure, done

Is there a way to have an option in code_checks.sh that actually runs the appropriate pre-commit hook?

Have opened #36956 for this

[jreback]

lgtm. can you merge master. merge on green (just to be sure nothing broken since this).

[jreback]

thanks @MarcoGorelli