Set polymorphic.__version__ from setuptools metadata

polymorphic/__init__.py

@@ -7,5 +7,7 @@ This code and affiliated files are (C) by Bert Constantin and individual contrib
 Please see LICENSE and AUTHORS for more information.
 """
 
-# See PEP 440 (https://www.python.org/dev/peps/pep-0440/)
+import pkg_resources
-__version__ = "1.3"
+
+
+__version__ = pkg_resources.require("django-polymorphic")[0].version