Bug 1600432: [taskgraph] Don't read `.taskcluster.yml` when loading the test; r=mtabara

The documentation tasks do not include .taskcluster.yml, so fail when trying to
load the tests to generate documentation automatically.

Differential Revision: https://phabricator.services.mozilla.com/D57054

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

taskcluster/taskgraph/test/test_taskcluster_yml.py

@@ -18,7 +18,9 @@ from taskgraph import GECKO
 
 class TestTaskclusterYml(unittest.TestCase):
 
-    taskcluster_yml = load_yaml(GECKO, ".taskcluster.yml")
+    @property
+    def taskcluster_yml(self):
+        return load_yaml(GECKO, ".taskcluster.yml")
 
     def test_push(self):
         context = {