Test the PURE_PYTHON environment variable support under tox.

CHANGES.rst

 ``BTrees`` Changelog
 ====================
 
+4.0.7 (unreleased)
+------------------
+
+- Test ``PURE_PYTHON`` environment variable support:  if set, the C
+  extensions will not be built, imported, or tested.
+
 
 4.0.6 (2013-05-14)
 ------------------

tox.ini

@@ -3,7 +3,7 @@ envlist =
 # Jython support pending 2.7 support, due 2012-07-15 or so.  See:
 # http://fwierzbicki.blogspot.com/2012/03/adconion-to-fund-jython-27.html
 #   py26,py27,py32,jython,pypy,coverage,docs
-    py26,py27,pypy,py32,py33,w_zodb,coverage,docs
+    py26,py27,py27-pure,pypy,py32,py33,w_zodb,coverage,docs
 
 [testenv]
 deps =
@@ -13,6 +13,16 @@ deps =
 commands =
     python setup.py test -q
 
+[testenv:py27-pure]
+basepython =
+    python2.7
+setenv =
+    PURE_PYTHON = 1
+deps =
+    {[testenv]deps}
+commands =
+    python setup.py test -q
+
 #[testenv:jython]
 #commands =
 #   jython setup.py test -q