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/hashlib.rst b/Doc/library/hashlib.rst
index 4a8d705..a16c7cd 100644
--- a/Doc/library/hashlib.rst
+++ b/Doc/library/hashlib.rst
@@ -271,7 +271,7 @@
    factor and *maxmem* limits memory (OpenSSL 1.1.0 defaults to 32 MiB).
    *dklen* is the length of the derived key.
 
-   Availability: OpenSSL 1.1+
+   .. availability:: OpenSSL 1.1+.
 
    .. versionadded:: 3.6