Change UnicodeDecodeError objects so that the 'object' attribute
is a bytes object.

Add 'y' and 'y#' format specifiers that work like 's' and 's#'
but only accept bytes objects.
diff --git a/Doc/ext/extending.tex b/Doc/ext/extending.tex
index 2af88b5..1f3e2d5 100644
--- a/Doc/ext/extending.tex
+++ b/Doc/ext/extending.tex
@@ -802,8 +802,10 @@
     Py_BuildValue("i", 123)                  123
     Py_BuildValue("iii", 123, 456, 789)      (123, 456, 789)
     Py_BuildValue("s", "hello")              'hello'
+    Py_BuildValue("y", "hello")              b'hello'
     Py_BuildValue("ss", "hello", "world")    ('hello', 'world')
     Py_BuildValue("s#", "hello", 4)          'hell'
+    Py_BuildValue("y#", "hello", 4)          b'hell'
     Py_BuildValue("()")                      ()
     Py_BuildValue("(i)", 123)                (123,)
     Py_BuildValue("(ii)", 123, 456)          (123, 456)