CI: flake8 check does not work properly

[charlesdong1991]

xref #37938

Have seen it in #37923 and in #37703 , so ci checks ran successfully in those feature PRs, but actually they should have detected this unused import.

Basically flake8 check in CI should detect cases such as unused imports, however, right now, observed from those two PRs, it didn't detect them in feature PRs and then resulted in CI failure on master