Fix compilation of "ParseTreeTransforms" in Py2. It previously tried to...

Fix compilation of "ParseTreeTransforms" in Py2. It previously tried to compare the 'directivetype' to 'str', which misses Py2 str when compiling with language level 3.

Cython/Compiler/ParseTreeTransforms.pxd

-# cython: language_level=3
+# cython: language_level=3str
 
 cimport cython
 

Cython/Compiler/ParseTreeTransforms.py

+# cython: language_level=3str
+
 from __future__ import absolute_import
 
 import cython