add link to flake8 in history

Author: dhellmann

docs/source/history.rst

@@ -15,7 +15,9 @@ dev
     caused by that rotating behavior, especially when the wrappers are
     being used by users with different permissions and
     umasks. (:bbissue:`152`)
-  - Use flake8 for style checking.
+  - Use flake8_ for style checking.
+
+.. _flake8: https://pypi.python.org/pypi/flake8
 
 3.6.1