Add functions PyUnicode_Append() and PyUnicode_AppendAndDel() that mirror
PyString_Concat() and PyString_ConcatAndDel() (the name PyUnicode_Concat()
was already taken).

Change PyObject_Repr() to always return a unicode object.

Update all repr implementations to return unicode objects.

Add a function PyObject_ReprStr8() that calls PyObject_Repr() and converts
the result to an 8bit string.

Use PyObject_ReprStr8() where using PyObject_Repr() can't be done
straightforward.
diff --git a/Objects/bufferobject.c b/Objects/bufferobject.c
index eb02e32..dd25668 100644
--- a/Objects/bufferobject.c
+++ b/Objects/bufferobject.c
@@ -319,13 +319,13 @@
 	const char *status = self->b_readonly ? "read-only" : "read-write";
 
 	if ( self->b_base == NULL )
-		return PyString_FromFormat("<%s buffer ptr %p, size %zd at %p>",
+		return PyUnicode_FromFormat("<%s buffer ptr %p, size %zd at %p>",
 					   status,
 					   self->b_ptr,
 					   self->b_size,
 					   self);
 	else
-		return PyString_FromFormat(
+		return PyUnicode_FromFormat(
 			"<%s buffer for %p, size %zd, offset %zd at %p>",
 			status,
 			self->b_base,