Workaround RTD error to generate the API pages

polymorphic/__init__.py

@@ -9,5 +9,8 @@ Please see LICENSE and AUTHORS for more information.
 
 import pkg_resources
 
+try:
+    __version__ = pkg_resources.require("django-polymorphic")[0].version
+except pkg_resources.DistributionNotFound:
+    __version__ = None  # for RTD among others
 
-__version__ = pkg_resources.require("django-polymorphic")[0].version