bpo-13601: always use line-buffering for sys.stderr

Add versionchanged note to docs.
python · jendrikseipp · jendrikseipp · jendrikseipp · Dec 17, 2019 · the-knights-who-say-ni
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
+     the :option:`-u` command-line option.
+
+   .. versionchanged:: 3.9
+      Use line-buffering for non-interactive ``stderr``.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
+   * When interactive, the ``stdout`` stream is line-buffered. Otherwise,
+     it is block-buffered like regular text files.  The ``stderr`` stream
+     is line-buffered in both cases.  You can override this behaviour with
+     the :option:`-u` command-line option.