prevent Py3.x build from picking up outdated sources from a previous 2to3 run

setup.py

@@ -175,10 +175,13 @@ def compile_cython_modules(profile=False, compile_more=False, cython_with_refnan
             dead_modules = []
 
             def build_extensions(self):
+                if sys.version_info[:2] == (3, 2):
                     # add path where 2to3 installed the transformed sources
                     # and make sure Python (re-)imports them from there
-                already_imported = [ module for module in sys.modules
-                                     if module == 'Cython' or module.startswith('Cython.') ]
+                    already_imported = [
+                        module for module in sys.modules
+                        if module == 'Cython' or module.startswith('Cython.')
+                    ]
                     keep_alive = self.dead_modules.append
                     for module in already_imported:
                         keep_alive(sys.modules[module])