CI unpin flake8, only run flake8-rst in pre-commit
- [ ] closes #34150 (maybe?)
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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