Simplify isort filters and move it into isort cfg file.
With this change, isort will not longer apply to diffs other than to files that are in whitelisted directory (isort only supports blacklist so we implement that instead) This is much simpler than building our own whitelist logic since our formatter runs multiple codepaths depending on whether it is formatting a single file / PR / entire repo in CI.
It will be easier to develop if we could use a tool to organize / sort imports and not have to move them around by hand.
This PR shows how we could do this with isort (black doesn't quite do this per https://github.com/psf/black/issues/333)
After this PR lands everyone will need to update their formatter to include isort if they don't have it already, i.e.
pip install -r ./python/requirements_linters.txt
All future file changes will go through isort and may introduce a slightly larger PR the first time as it will clean up the imports.
The plan is to land this PR and also clean up the rest of the code in parallel by using this PR to format the codebase (so people won't get surprised by the formatter if the file hasn't been touched yet)
Co-authored-by: Clarence Ng <clarence@anyscale.com>
