Test w/ PURE_PYTHON set under Python3.5.

tox.ini

@@ -3,7 +3,7 @@ envlist =
 # Jython 2.7rc2 does work, but unfortunately has an issue running
 # with Tox 1.9.2 (http://bugs.jython.org/issue2325)
 #   py27,py27-pure,pypy,py33,py34,pypy3,jython,coverage,docs
-    py27,py27-pure,py27-pure-cffi,pypy,py33,py34,py35,pypy3,coverage,docs
+    py27,py27-pure,py27-pure-cffi,pypy,py33,py34,py35,py35-pure,pypy3,coverage,docs
 
 [testenv]
 deps =
@@ -39,6 +39,17 @@ deps =
 commands =
     python setup.py -q test -q
 
+[testenv:py35-pure]
+basepython =
+    python3.5
+setenv =
+    PURE_PYTHON = 1
+    PIP_CACHE_DIR = {envdir}/.cache
+deps =
+    {[testenv]deps}
+commands =
+    python setup.py -q test -q
+
 
 [testenv:coverage]
 basepython =