[2.0.x] Fixed serializers test crash if PyYAML isn't installed.

Follow up to a57c783d.

Backport of 55490ac7 from master

tests/serializers/test_yaml.py

@@ -116,7 +116,7 @@ class YamlSerializerTestCase(SerializersTestBase, TestCase):
     headline: Poker has no place on ESPN
     pub_date: 2006-06-16 11:00:00
     categories:""" + (
-        ' [%(first_category_pk)s, %(second_category_pk)s]' if yaml.__version__ < '5.1'
+        ' [%(first_category_pk)s, %(second_category_pk)s]' if HAS_YAML and yaml.__version__ < '5.1'
         else '\n    - %(first_category_pk)s\n    - %(second_category_pk)s') + """
     meta_data: []
 """