Split out cmd_output_b

tests/languages/docker_test.py

@@ -9,7 +9,7 @@ from pre_commit.util import CalledProcessError
 
 def test_docker_is_running_process_error():
     with mock.patch(
-        'pre_commit.languages.docker.cmd_output',
+        'pre_commit.languages.docker.cmd_output_b',
         side_effect=CalledProcessError(*(None,) * 4),
     ):
         assert docker.docker_is_running() is False