[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/asyncio-stream.rst b/Doc/library/asyncio-stream.rst
index c543aa6..42b4c1f 100644
--- a/Doc/library/asyncio-stream.rst
+++ b/Doc/library/asyncio-stream.rst
@@ -115,7 +115,7 @@
 
    See also the documentation of :meth:`loop.create_unix_connection`.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -137,7 +137,7 @@
 
    See also the documentation of :meth:`loop.create_unix_server`.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7