Added Python 3 support to setup.py

Ticket #150 and commit 2f71d6621d7d1d7fe733488f09d23533c772c849 indicate that watchdog now supports Python 3.

This change updates the setup.py so that the next PyPI release will be properly marked as Python 3 compatible.
Merge pull request #207 from tomchristie/patch-1

Added Python 3 support to setup.py