Add functionality to skip files during isort #335

Job	Run time
static	1m 11s
test (docs)	3m 7s
test (examples)	2m 36s
test (test, test local var val, --test-arg --test-arg)	1m 19s
test (remote, 0)	6m 26s
test (remote-test, remote-test, 1)	6m 26s
test (test-coverage, test local var val, --test-arg, test-coverage)	1m 27s
test (test-coverage, test local var val, --test-arg, 3.8, ubuntu-20.04, test-coverage-oldest)	1m 12s
test (test-nengo, test local var val, --test-arg, 3.11)	2m 41s
coverage	22s
auto-update	0s
deploy	0s
	26m 47s

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