Update getting_started.rst

To use techniques like these: https://github.com/edx/course-discovery/blob/master/course_discovery/apps/api/serializers.py#L548 you need at least Python 3.5.

docs/getting_started.rst

 Getting Started
 ===============
 
-If you have not already done so, create/activate a `virtualenv`_ running Python 3. Unless otherwise stated, assume all terminal code
+If you have not already done so, create/activate a `virtualenv`_ running Python 3.5. Unless otherwise stated, assume all terminal code
 below is executed within the virtualenv.
 
 .. _virtualenv: https://virtualenvwrapper.readthedocs.org/en/latest/