Set language_level=2 in cythonize()

This suppresses a frequent warning about the future from Cython.

[rharwood@redhat.com: rewrote commit message]

Author: aiudirog

setup.py

@@ -213,7 +213,10 @@ def ext_modules(self):
             return self._ext_modules
 
         if getattr(self, '_cythonized_ext_modules', None) is None:
-            self._cythonized_ext_modules = cythonize(self._ext_modules)
+            self._cythonized_ext_modules = cythonize(
+                self._ext_modules,
+                language_level=2,
+            )
 
         return self._cythonized_ext_modules