[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/hashlib.rst b/Doc/library/hashlib.rst
index baf6b0a..ab4414f 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