make FileIO.mode always include 'b'

#4386 Reviewed by Amaury
diff --git a/Misc/NEWS b/Misc/NEWS
index 1e61e54..4e797b2 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -48,6 +48,8 @@
 Library
 -------
 
+- FileIO's mode attribute now always includes ``"b"``.
+
 - Issue #3799: Fix dbm.dumb to accept strings as well as bytes for keys. String
   keys are now written out in UTF-8.