restrict 2to3 usage to Python 3.2 (which does not support the u'' string literal prefix)

2to3-fixers.txt

-lib2to3.fixes.fix_metaclass
 lib2to3.fixes.fix_unicode

runtests.py

@@ -1764,9 +1764,9 @@ def main():
 
     WORKDIR = os.path.abspath(options.work_dir)
 
-    if sys.version_info[0] >= 3:
-        if options.with_cython:
+    if options.with_cython and sys.version_info[0] >= 3:
         sys.path.insert(0, options.cython_dir)
+        if sys.version_info[:2] == (3, 2):
             try:
                 # try if Cython is installed in a Py3 version
                 import Cython.Compiler.Main

setup.py

@@ -34,7 +34,7 @@ class sdist(sdist_orig):
         sdist_orig.run(self)
 add_command_class('sdist', sdist)
 
-if sys.version_info[0] >= 3:
+if sys.version_info[:2] == (3, 2):
     import lib2to3.refactor
     from distutils.command.build_py \
          import build_py_2to3 as build_py