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

Add What's new entry for datetime.fromisoformat (GH-5559) (GH-5939) · python/cpython@0e06be8

Original file line number	Diff line number	Diff line change
`@@ -502,6 +502,14 @@ Added support for the Blowfish method.`
`502`	`502`	The :func:`~crypt.mksalt` function now allows to specify the number of rounds
`503`	`503`	for hashing. (Contributed by Serhiy Storchaka in :issue:`31702`.)
`504`	`504`
	`505`	`+datetime`
	`506`	`+--------`
	`507`	`+`
	`508`	+Added the :func:`datetime.datetime.fromisoformat` method, which constructs a
	`509`	+:class:`datetime.datetime` object from a string in one of the formats output
	`510`	+by :func:`datetime.datetime.isoformat`. (Contributed by Paul Ganssle in
	`511`	+:issue:`15873`.)
	`512`	`+`
`505`	`513`	`dis`
`506`	`514`	`---`
`507`	`515`