bpo-11233: Create availability directive for documentation (GH-9692)

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

Co-Authored-By: Georg Brandl <georg@python.org>
diff --git a/Doc/library/select.rst b/Doc/library/select.rst
index 8feedaa..f5b25db 100644
--- a/Doc/library/select.rst
+++ b/Doc/library/select.rst
@@ -171,7 +171,9 @@
    :func:`poll` or another interface in this module.  This doesn't apply
    to other kind of file-like objects such as sockets.
 
-   This value is guaranteed by POSIX to be at least 512.  Availability: Unix.
+   This value is guaranteed by POSIX to be at least 512.
+
+   .. availability:: Unix
 
    .. versionadded:: 3.2