[3.7] bpo-11233: Create availability directive for documentation (GH-9692) (GH-9830)

Replace "Availability: xxx" with ".. availability:: xxx" in the doc.
 Original patch by Georg Brandl.

 Co-Authored-By: Georg Brandl <georg@python.org>
 (cherry picked from commit 2d6097d027e0dd3debbabc702aa9c98d94ba32a3)

 Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
diff --git a/Doc/library/shutil.rst b/Doc/library/shutil.rst
index 1527deb..382f75d 100644
--- a/Doc/library/shutil.rst
+++ b/Doc/library/shutil.rst
@@ -323,7 +323,7 @@
 
    .. versionadded:: 3.3
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
 .. function:: chown(path, user=None, group=None)
 
@@ -334,7 +334,7 @@
 
    See also :func:`os.chown`, the underlying function.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.3