BLD: pin cython language level to '2'

This is a simpler alternative to PyWavelets#435

Author: tacaswell

setup.py

@@ -163,7 +163,7 @@ def get_cython_sources(use_cython):
         fd.write('%s = %d\n' % (k, int(v)))
 
 
-cythonize_opts = {}
+cythonize_opts = {'language_level': '2'}
 if os.environ.get("CYTHON_TRACE"):
     cythonize_opts['linetrace'] = True
     cython_macros.append(("CYTHON_TRACE_NOGIL", 1))