Add versionadded for hmac.secure_compare().

Doc/library/hmac.rst

@@ -83,6 +83,7 @@ This module also provides the following helper function:
       contents of the inputs via a timing attack, it does leak the length
       of the inputs. However, this generally is not a security risk.
 
+   .. versionadded:: 3.3
 
 .. seealso::