add a tox env for Python 3.3

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, pypy, precise, trunk
+envlist = py27, pypy, precise, trunk, py33
 
 [testenv]
 deps =
@@ -37,6 +37,20 @@ commands =
     pip install https://github.com/scrapy/queuelib/archive/master.zip#egg=queuelib
     trial scrapy
 
+[testenv:py33]
+basepython = python3.3
+deps =
+    Twisted>=13.2.0
+    lxml>=3.2.4
+    pyOpenSSL>=0.13.1
+    cssselect>=0.9
+    queuelib>=1.1.1
+    w3lib>=1.5
+    # Only required to run tests
+    mock>=1.0.1
+commands =
+    trial scrapy
+
 [testenv:windows]
 commands =
     bin/runtests.bat []