[python linting] tweak options

Summary:
There's no need to specify --select when we're picking all the options.

It turns out that autopep8 with W50x options is a bit buggy, so disable that for now.
https://github.com/hhatto/autopep8/issues/518

Test Plan:

arc lint --everything

(tried on flake8 3.5.0, 3.6.0, and 3.7.9 latest)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5219