[autopep8] set max line length to 88

Summary:
This is becoming a new de-facto standard for recent python tools (flynt, black...), so it will allow me to introduce flynt as a an auto-formatting tool without having to tweak its line-lenght behavior, and maybe in the future add also black as a fomatting tool.

The upsides are:

• less weird autopep8 line breaks
• slightly less merge conflicts when backporting

Test Plan: arc lint --everything

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13190