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-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 76c8ce9..96f5e0b 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -510,7 +510,7 @@
    See the documentation of the :meth:`loop.create_connection` method
    for information about arguments to this method.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -630,7 +630,7 @@
    See the documentation of the :meth:`loop.create_server` method
    for information about arguments to this method.
 
-   Availability: Unix.
+   .. availability:: Unix.
 
    .. versionadded:: 3.7
 
@@ -979,7 +979,7 @@
    Return ``True`` if the signal handler was removed, or ``False`` if
    no handler was set for the given signal.
 
-Availability: Unix.
+   .. availability:: Unix.
 
 .. seealso::
 
@@ -1423,14 +1423,14 @@
       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.
 
    .. seealso::