bpo-36597: fix weakref example code (GH-12779)

Commit 57b1a286 fixed doctest, but example code is not
match with document.
Just skip doctest for the block.

Doc/library/weakref.rst

@@ -489,10 +489,13 @@ Unless you set the :attr:`~finalize.atexit` attribute to
 :const:`False`, a finalizer will be called when the program exits if it
 is still alive.  For instance
 
+.. doctest::
+   :options: +SKIP
+
    >>> obj = Object()
-    >>> weakref.finalize(obj, print, "obj dead or exiting")  #doctest:+ELLIPSIS
+   >>> weakref.finalize(obj, print, "obj dead or exiting")
    <finalize object at ...; for 'Object' at ...>
-    >>> del obj
+   >>> exit()
    obj dead or exiting