Rely on dulwich <= 0.9.5 for python2.6 tests

tox.ini

@@ -14,7 +14,7 @@ commands = nosetests []
 [testenv:py26]
 deps =
   python-ldap
-  dulwich
+  dulwich<=0.9.5
   {[base]deps}
 
 [testenv:py27]