Add What's new entry for datetime.fromisoformat (GH-5559) (GH-5939)

Documents bpo-15873
(cherry picked from commit 22864bc)

Co-authored-by: Paul Ganssle <pganssle@users.noreply.github.com>

Doc/whatsnew/3.7.rst

@@ -502,6 +502,14 @@ Added support for the Blowfish method.
 The :func:`~crypt.mksalt` function now allows to specify the number of rounds
 for hashing.  (Contributed by Serhiy Storchaka in :issue:`31702`.)
 
+datetime
+--------
+
+Added the :func:`datetime.datetime.fromisoformat` method, which constructs a
+:class:`datetime.datetime` object from a string in one of the formats output
+by :func:`datetime.datetime.isoformat`. (Contributed by Paul Ganssle in
+:issue:`15873`.)
+
 dis
 ---