Merge pull request #18310 from edx/jmbowman/TE-2543

TE-2543 Add tox env for Python 3.5

tox.ini

 [tox]
-envlist = py27-django{18,19,110,111}
+envlist = py{27,35}-django{18,19,110,111}
 
 # This is needed to prevent the lms, cms, and openedx packages inside the "Open
 # edX" package (defined in setup.py) from getting installed into site-packages