Move the "b" function to pycompat.

tests/test_markup.py

@@ -10,7 +10,6 @@
 """
 
 import re
-import sys
 
 from util import *
 
@@ -18,15 +17,10 @@ from docutils import frontend, utils, nodes
 from docutils.parsers import rst
 
 from sphinx.util import texescape
+from sphinx.util.pycompat import b
 from sphinx.writers.html import HTMLWriter, SmartyPantsHTMLTranslator
 from sphinx.writers.latex import LaTeXWriter, LaTeXTranslator
 
-if sys.version_info > (3, 0):
-    def b(s):
-        return s.encode('utf-8')
-else:
-    b = str
-
 def setup_module():
     global app, settings, parser
     texescape.init()  # otherwise done by the latex builder