Lint everything

Summary:
With updated python linting tools, one stylistic change and one real
issue found by the linter (the format() one).

Test Plan:

ninja check-functionl

Should have no error:

arc lint --everything

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6118