Link to single source version was broken.

Author: Thomas Güttler

setup.py

@@ -13,7 +13,7 @@
 
     # Versions should comply with PEP440.  For a discussion on single-sourcing
     # the version across setup.py and the project code, see
-    # https://packaging.python.org/en/latest/development.html#single-sourcing-the-version
+    # https://packaging.python.org/en/latest/single_source_version.html
     version='1.2.0',
 
     description='A sample Python project',