[linter] Make flake8 version >= 5 happy

Summary: The latest flake8 version expects the keywords to have no parenthesis (e.g. assert foo vs assert(foo) or a space (e.g. assert (foo)) if needed.

Test Plan:
With flake8 version 5.0.0:

arc lint --everything

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11911