Use 'io.BytesIO' rather than 'cStringIO.StringIO'.

python/google/protobuf/internal/encoder.py

@@ -43,7 +43,7 @@ FieldDescriptor) we construct two functions:  a "sizer" and an "encoder".  The
 sizer takes a value of this field's type and computes its byte size.  The
 encoder takes a writer function and a value.  It encodes the value into byte
 strings and invokes the writer function to write those strings.  Typically the
-writer function is the write() method of a cStringIO.
+writer function is the write() method of a BytesIO.
 
 We try to do as much work as possible when constructing the writer and the
 sizer rather than when calling them.  In particular:

python/google/protobuf/text_format.py

@@ -34,7 +34,7 @@
 
 __author__ = 'kenton@google.com (Kenton Varda)'
 
-import cStringIO
+import io
 import re
 
 import six
@@ -89,7 +89,7 @@ def MessageToString(message, as_utf8=False, as_one_line=False,
   Returns:
     A string of the text formatted protocol buffer message.
   """
-  out = cStringIO.StringIO()
+  out = io.BytesIO()
   PrintMessage(message, out, as_utf8=as_utf8, as_one_line=as_one_line,
                pointy_brackets=pointy_brackets,
                use_index_order=use_index_order,