Replaced cStringIO.StringIO by io.BytesIO.

Also replaced StringIO.StringIO by BytesIO in some other appropriate
places. StringIO is not available in Python 3.

django/utils/text.py

@@ -3,11 +3,7 @@ import unicodedata
 import warnings
 from gzip import GzipFile
 from htmlentitydefs import name2codepoint
-
-try:
-    from cStringIO import StringIO
-except ImportError:
-    from StringIO import StringIO
+from io import BytesIO
 
 from django.utils.encoding import force_unicode
 from django.utils.functional import allow_lazy, SimpleLazyObject
@@ -277,7 +273,7 @@ phone2numeric = allow_lazy(phone2numeric)
 # From http://www.xhaus.com/alan/python/httpcomp.html#gzip
 # Used with permission.
 def compress_string(s):
-    zbuf = StringIO()
+    zbuf = BytesIO()
     zfile = GzipFile(mode='wb', compresslevel=6, fileobj=zbuf)
     zfile.write(s)
     zfile.close()