Update Django Guardian dependency. (#6430)

* Update Django Guardian dependency.
* Skip testing Guardian on PY2. See django-guardian/django-guardian#602

Author: carltongibson

requirements/requirements-optionals.txt

@@ -1,7 +1,7 @@
 # Optional packages which may be used with REST framework.
 psycopg2-binary==2.7.5
 markdown==2.6.11
-django-guardian==1.4.9
+django-guardian==1.5.0
 django-filter==1.1.0
 coreapi==2.3.1
 coreschema==0.0.4

rest_framework/compat.py

@@ -161,6 +161,10 @@ def is_guardian_installed():
     """
     django-guardian is optional and only imported if in INSTALLED_APPS.
     """
+    if six.PY2:
+        # Guardian 1.5.0, for Django 2.2 is NOT compatible with Python 2.7.
+        # Remove when dropping PY2.
+        return False
     return 'guardian' in settings.INSTALLED_APPS