Add functionality to skip files during isort #336

Job	Run time
static	1m 5s
test (docs)	3m 9s
test (examples)	2m 28s
test (test, test local var val, --test-arg --test-arg)	1m 5s
test (remote, 0)	2m 51s
test (remote-test, remote-test, 1)	3m 30s
test (test-coverage, test local var val, --test-arg, test-coverage)	1m 18s
test (test-coverage, test local var val, --test-arg, 3.8, ubuntu-20.04, test-coverage-oldest)	1m 30s
test (test-nengo, test local var val, --test-arg, 3.11)	1m 24s
auto-update	0s
coverage	21s
deploy	0s
	18m 41s

Provide feedback