Add functionality to skip files during isort #337

Job	Run time
static	1m 13s
test (docs)	3m 5s
test (examples)	2m 41s
test (test, test local var val, --test-arg --test-arg)	1m 13s
test (remote, 0)	3m 28s
test (remote-test, remote-test, 1)	3m 30s
test (test-coverage, test local var val, --test-arg, test-coverage)	1m 28s
test (test-coverage, test local var val, --test-arg, 3.8, ubuntu-20.04, test-coverage-oldest)	1m 22s
test (test-nengo, test local var val, --test-arg, 3.11)	1m 21s
auto-update	2m 18s
coverage	16s
deploy	0s
	21m 55s

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