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/asyncio-stream.rst b/Doc/library/asyncio-stream.rst
index 60aae16..e686a6a 100644
--- a/Doc/library/asyncio-stream.rst
+++ b/Doc/library/asyncio-stream.rst
@@ -114,7 +114,7 @@
 
    See also the documentation of :meth:`loop.create_unix_connection`.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -136,7 +136,7 @@
 
    See also the documentation of :meth:`loop.create_unix_server`.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7