Add functionality to skip files during isort #331

Job	Run time
static	1m 18s
test (docs)	1m 14s
test (examples)	2m 38s
test (test, test local var val, --test-arg --test-arg)	1m 5s
test (remote, 0)	7m 58s
test (remote-test, remote-test, 1)	7m 58s
test (test-coverage, test local var val, --test-arg, test-coverage)	1m 17s
test (test-coverage, test local var val, --test-arg, 3.8, ubuntu-20.04, test-coverage-oldest)	1m 10s
test (test-nengo, test local var val, --test-arg, 3.11)	1m 34s
coverage	22s
auto-update	0s
deploy	0s
	26m 34s

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