add GitHub URL for PyPi

Author: andriyor

setup.py

@@ -161,6 +161,9 @@
       author=AUTHOR,
       author_email=AUTHOR_EMAIL,
       url=URL,
+      project_urls={
+          'Source': 'https://github.com/PythonCharmers/python-future',
+      },
       description=DESCRIPTION,
       long_description=LONG_DESC,
       license=LICENSE,