bpo-37330: open(): "U" mode is removed in Python 3.9 (GH-16972)

Update open() documentation.
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 0236ff0..bc46844 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1245,7 +1245,7 @@
 
          * The file is now non-inheritable.
 
-   .. deprecated-removed:: 3.4 4.0
+   .. deprecated-removed:: 3.4 3.9
 
       The ``'U'`` mode.