Testing info in README, wider Python compatibility

The library has now been tested with Python 2.7, 3.3, 3.4, 3.5, and 3.6.
The README and setup.py now indicate this.  The README also says how to
run the tests, and that you need a PD Buddy Sink plugged in for them to
be completely thorough.

README.rst

@@ -55,6 +55,17 @@ Write the SinkConfig object to flash
 Requirements
 ------------
 
--  Python 2.7, 3.6
+-  Python 2.7, >= 3.3
 -  pySerial >= 3.0
 -  aenum >= 2.0 (if using Python < 3.6)
+
+Testing
+-------
+
+To run the unit tests, run::
+
+    $ python setup.py test
+
+This will test the Sink class only if a PD Buddy Sink is plugged in and in
+setup mode, so make sure that's the case if you're testing any changes to the
+Sink class.

setup.py

@@ -24,6 +24,9 @@ setup(
         "Operating System :: Microsoft :: Windows",
         "Programming Language :: Python",
         "Programming Language :: Python :: 2.7",
+        "Programming Language :: Python :: 3.3",
+        "Programming Language :: Python :: 3.4",
+        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Topic :: Software Development :: User Interfaces"
     ],