This reverts r63675 based on the discussion in this thread:

http://mail.python.org/pipermail/python-dev/2008-June/079988.html

Python 2.6 should stick with PyString_* in its codebase.  The PyBytes_* names
in the spirit of 3.0 are available via a #define only.  See the email thread.

Objects/stringlib/string_format.h

@@ -496,7 +496,7 @@ render_field(PyObject *fieldobj, SubString *format_spec, OutputString *output)
 #if PY_VERSION_HEX >= 0x03000000
     assert(PyUnicode_Check(result));
 #else
-    assert(PyBytes_Check(result) || PyUnicode_Check(result));
+    assert(PyString_Check(result) || PyUnicode_Check(result));
 
     /* Convert result to our type.  We could be str, and result could
        be unicode */