bpo-45447: Add entry to What's new 3.9 (GH-31305)

Author: terryjreedy

Doc/whatsnew/3.9.rst

@@ -500,6 +500,9 @@ Reedy in :issue:`40468`.)  Move the indent space setting from the Font tab to
 the new Windows tab.  (Contributed by Mark Roseman and Terry Jan Reedy in
 :issue:`33962`.)
 
+Apply syntax highlighting to `.pyi` files. (Contributed by Alex
+Waygood and Terry Jan Reedy in :issue:`45447`.)
+
 imaplib
 -------