apply flake8 to tests #8836

Job	Run time
1/4	12m 59s
2/4	13m 0s
3/4	11m 6s
4/4	16m 26s
py-3-latest	20m 56s
flaky	6m 13s
_remote_background_indep_poll	5m 14s
_remote_background_indep_tcp	7m 9s
macos 1/5	24m 33s
macos 2/5	21m 9s
codecov	10s
	2h 18m 55s

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