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/Python/errors.c b/Python/errors.c
index 443235a..4ef491f 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -54,7 +54,7 @@
 {
 	if (exception != NULL &&
 	    !PyExceptionClass_Check(exception)) {
-		PyObject *excstr = PyObject_Repr(exception);
+		PyObject *excstr = PyObject_ReprStr8(exception);
 		PyErr_Format(PyExc_SystemError,
 			     "exception %s not a BaseException subclass",
 			     PyString_AS_STRING(excstr));