Compile Cython's modules with language_level=3/3str.

Cython/Compiler/Code.py

-# cython: language_level = 2
+# cython: language_level=3str
 # cython: auto_pickle=False
 #
 #   Code output module

Cython/Compiler/FlowControl.py

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

Cython/Compiler/Scanning.py

-# cython: infer_types=True, language_level=3, py2_import=True, auto_pickle=False
+# cython: infer_types=True, language_level=3, auto_pickle=False
 #
 #   Cython Scanner
 #

Cython/Plex/Actions.py

+# cython: language_level=3str
 # cython: auto_pickle=False
 #=======================================================================
 #

Cython/Plex/Scanners.py

+# cython: language_level=3str
 # cython: auto_pickle=False
 #=======================================================================
 #

Cython/Tempita/_tempita.py

+# cython: language_level=3str
+
 """
 A small templating language