Prefer 2.7 over 2.6.

tox.ini

@@ -29,7 +29,7 @@ commands =
 
 [testenv:w_zodb]
 basepython =
-    python2.6
+    python2.7
 commands =
     python setup.py test -q
 deps =
@@ -43,7 +43,7 @@ deps =
 
 [testenv:coverage]
 basepython =
-    python2.6
+    python2.7
 commands =
     nosetests --with-xunit --with-xcoverage --cover-package=BTrees
 deps =
@@ -56,7 +56,7 @@ deps =
 
 [testenv:docs]
 basepython =
-    python2.6
+    python2.7
 commands =
     sphinx-build -b html -d docs/_build/doctrees docs docs/_build/html
     sphinx-build -b doctest -d docs/_build/doctrees docs docs/_build/doctest