Add method quote_plus in module py3kcompat.urlutils

Method quote_plus is used in some projects. We need add this method
to work with Python 3. This is an example:
https://github.com/stackforge/stackalytics/blob/master/dashboard/helpers.py#L174

Change-Id: I15ed496d947cf2b56eb6645b6b85c0745ab4f682
Merge "Add method quote_plus in module py3kcompat.urlutils"