Bug 1532783: [taskgraph] Fix lint error.

Differential Revision: https://phabricator.services.mozilla.com/D29438

--HG--
extra : moz-landing-system : lando

taskcluster/taskgraph/transforms/job/mozharness.py

@@ -245,7 +245,9 @@ def mozharness_on_docker_worker_setup(config, job, taskdesc):
 @run_job_using("generic-worker", "mozharness", schema=mozharness_run_schema,
                defaults=mozharness_defaults)
 def mozharness_on_generic_worker(config, job, taskdesc):
-    assert job['worker']['os'] == 'windows', 'only supports windows right now: {}'.format(job['label'])
+    assert (
+        job["worker"]["os"] == "windows"
+    ), "only supports windows right now: {}".format(job["label"])
 
     run = job['run']