STYLE pandas-dev flake8 plugin
Currently, there are a lot of style checks which are specific to this repo, the result of which is a ton of CI checks and occasionally quite a bit of extra code.

The main repo is already huge, so I'd like to suggest making a `pandas-dev/pandas-style-guide` repo, which would be a `flake8` plugin, with all the custom linting rules we currently have.

Some advantages:
- the current gigantic repo would become a bit smaller, as many style checks could be moved over to the `pandas-dev/pandas-style-guide` one
- the plugin could easily be tested on multiple OSs and versions of Python, which is currently difficult with all the stylistic checks living in the main repo. The `ast` module often changes a bit between versions, so it would be good to test the ast-based checks on different Python versions
- linting failures would be reported with the familiar flake8 syntax we're all accustomed to by now

One disadvantage I see is that there might be less visibility if this is in a separate repo.

@pandas-dev/pandas-core @pandas-dev/pandas-triage anyone have any thoughts/objections?