Fix Flake8 E741 errors

Summary:
These errors are added in newer versions of pycodestyle.
https://www.flake8rules.com/rules/E741.html

Test Plan:
With pycodestyle >= 2.6.0

arc lint --everything

Check there is no E741 error.

ninja check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6117