[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-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index a61b971..df453cd 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -504,7 +504,7 @@
    See the documentation of the :meth:`loop.create_connection` method
    for information about arguments to this method.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -624,7 +624,7 @@
    See the documentation of the :meth:`loop.create_server` method
    for information about arguments to this method.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -973,7 +973,7 @@
    Return ``True`` if the signal handler was removed, or ``False`` if
    no handler was set for the given signal.
 
-Availability: Unix.
+   .. availability:: Unix.
 
 .. seealso::
 
@@ -1417,14 +1417,14 @@
       asyncio.set_event_loop(loop)
 
 
-   Availability: Unix, Windows.
+   .. availability:: Unix, Windows.
 
 
 .. class:: ProactorEventLoop
 
    An event loop for Windows that uses "I/O Completion Ports" (IOCP).
 
-   Availability: Windows.
+   .. availability:: Windows.
 
    An example how to use :class:`ProactorEventLoop` on Windows::