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/Modules/_fileio.c b/Modules/_fileio.c
index d524aa7..2bda155 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -580,9 +580,9 @@
 fileio_repr(PyFileIOObject *self)
 {
         if (self->fd < 0)
-		return PyString_FromFormat("_fileio._FileIO(-1)");
+		return PyUnicode_FromFormat("_fileio._FileIO(-1)");
 
-	return PyString_FromFormat("_fileio._FileIO(%d, '%s')",
+	return PyUnicode_FromFormat("_fileio._FileIO(%d, '%s')",
 				   self->fd, mode_string(self));
 }