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.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 5e8e05d..371d8c1 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -151,7 +151,7 @@
 
 /*
  * Hacked up version of set_lookkey which can assume keys are always strings;
- * This means we can always use _PyBytes_Eq directly and not have to check to
+ * This means we can always use _PyString_Eq directly and not have to check to
  * see if the comparison altered the table.
  */
 static setentry *
@@ -168,7 +168,7 @@
 	   including subclasses of str; e.g., one reason to subclass
 	   strings is to override __eq__, and for speed we don't cater to
 	   that here. */
-	if (!PyBytes_CheckExact(key)) {
+	if (!PyString_CheckExact(key)) {
 		so->lookup = set_lookkey;
 		return set_lookkey(so, key, hash);
 	}
@@ -179,7 +179,7 @@
 	if (entry->key == dummy)
 		freeslot = entry;
 	else {
-		if (entry->hash == hash && _PyBytes_Eq(entry->key, key))
+		if (entry->hash == hash && _PyString_Eq(entry->key, key))
 			return entry;
 		freeslot = NULL;
 	}
@@ -194,7 +194,7 @@
 		if (entry->key == key
 		    || (entry->hash == hash
 			&& entry->key != dummy
-			&& _PyBytes_Eq(entry->key, key)))
+			&& _PyString_Eq(entry->key, key)))
 			return entry;
 		if (entry->key == dummy && freeslot == NULL)
 			freeslot = entry;
@@ -381,8 +381,8 @@
 	register long hash;
 	register Py_ssize_t n_used;
 
-	if (!PyBytes_CheckExact(key) ||
-	    (hash = ((PyBytesObject *) key)->ob_shash) == -1) {
+	if (!PyString_CheckExact(key) ||
+	    (hash = ((PyStringObject *) key)->ob_shash) == -1) {
 		hash = PyObject_Hash(key);
 		if (hash == -1)
 			return -1;
@@ -428,8 +428,8 @@
 	PyObject *old_key;
 
 	assert (PyAnySet_Check(so));
-	if (!PyBytes_CheckExact(key) ||
-	    (hash = ((PyBytesObject *) key)->ob_shash) == -1) {
+	if (!PyString_CheckExact(key) ||
+	    (hash = ((PyStringObject *) key)->ob_shash) == -1) {
 		hash = PyObject_Hash(key);
 		if (hash == -1)
 			return -1;
@@ -618,7 +618,7 @@
 	if (status != 0) {
 		if (status < 0)
 			return NULL;
-		return PyBytes_FromFormat("%s(...)", so->ob_type->tp_name);
+		return PyString_FromFormat("%s(...)", so->ob_type->tp_name);
 	}
 
 	keys = PySequence_List((PyObject *)so);
@@ -629,8 +629,8 @@
 	if (listrepr == NULL)
 		goto done;
 
-	result = PyBytes_FromFormat("%s(%s)", so->ob_type->tp_name,
-		PyBytes_AS_STRING(listrepr));
+	result = PyString_FromFormat("%s(%s)", so->ob_type->tp_name,
+		PyString_AS_STRING(listrepr));
 	Py_DECREF(listrepr);
 done:
 	Py_ReprLeave((PyObject*)so);
@@ -685,8 +685,8 @@
 	long hash;
 	setentry *entry;
 
-	if (!PyBytes_CheckExact(key) ||
-	    (hash = ((PyBytesObject *) key)->ob_shash) == -1) {
+	if (!PyString_CheckExact(key) ||
+	    (hash = ((PyStringObject *) key)->ob_shash) == -1) {
 		hash = PyObject_Hash(key);
 		if (hash == -1)
 			return -1;
@@ -983,7 +983,7 @@
 	register PySetObject *so = NULL;
 
 	if (dummy == NULL) { /* Auto-initialize dummy */
-		dummy = PyBytes_FromString("<dummy key>");
+		dummy = PyString_FromString("<dummy key>");
 		if (dummy == NULL)
 			return NULL;
 	}
@@ -2322,7 +2322,7 @@
 	/* Exercise direct iteration */
 	i = 0, count = 0;
 	while (_PySet_Next((PyObject *)dup, &i, &x)) {
-		s = PyBytes_AsString(x);
+		s = PyString_AsString(x);
 		assert(s && (s[0] == 'a' || s[0] == 'b' || s[0] == 'c'));
 		count++;
 	}