bpo-13601: Mention stderr's line buffering in What's New (GH-20168)

bpo-13601: Mention stderr's line buffering in What's New (GH-20168) · python/cpython@d17f3d8

Original file line number	Diff line number	Diff line change
@@ -542,6 +542,10 @@ most platforms. On Fedora and SuSE, it is equal to ``"lib64"`` on 64-bit
`542`	`542`	`platforms.`
`543`	`543`	(Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)
`544`	`544`
	`545`	+Previously, :attr:`sys.stderr` was block-buffered when non-interactive. Now
	`546`	+``stderr`` defaults to always being line-buffered.
	`547`	+(Contributed by Jendrik Seipp in :issue:`13601`.)
	`548`	`+`
`545`	`549`
`546`	`550`	`typing`
`547`	`551`	`------`