Fix changes script to properly ignore unwanted issues (incorporates Fixed chages to paginate API call. #513).

The script will now properly display only unlabeled issues, or issues with the labels bug, docs, enhancement, or tests. Also made some PEP8 related style changes.

Take a look at the CHANGEKIND_NAME dict, setting the value of any of the items in there will enable that kind of issue in the changelog, None is used to disable that kind of issue. The order the items appear in the dict determine the order they will appear in the change log. Unlabeled items are indicated by the value 'unlabeled', currently sorted to the top of the changelog.