[LINTER] Enforce a minimum version for autopep8

Summary:
The behavior of autopep8 differs between older and newer versions.
Enforcing a minimum version is an attempt to enforce the same rules for
everybody, avoiding code conflicts coming to the linter.

The minimum version is selected as being the minimum version shipped
with these OSs:

• Ubuntu 18.04 (Ubuntu 16.04 version is older but can be easily

updated)

• Debian testing
• FreeBSD 12.0
• OSX 10.12 Sierra

Test Plan:
Should return no error:

arc lint -- test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2754