bpo-45447: Add entry to What's new 3.10 (GH-31304)

(cherry picked from commit 2d98433)

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: miss-islington

Doc/whatsnew/3.10.rst

@@ -1148,7 +1148,7 @@ 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`.)
 
-These changes were backported to a 3.9 maintenance release.
+The changes above were backported to a 3.9 maintenance release.
 
 Add a Shell sidebar.  Move the primary prompt ('>>>') to the sidebar.
 Add secondary prompts ('...') to the sidebar.  Left click and optional
@@ -1170,6 +1170,11 @@ pattern-matching statements. However, this highlighting is not perfect
 and will be incorrect in some rare cases, including some ``_``-s in
 ``case`` patterns.  (Contributed by Tal Einat in :issue:`44010`.)
 
+New in 3.10 maintenance releases.
+
+Apply syntax highlighting to `.pyi` files. (Contributed by Alex
+Waygood and Terry Jan Reedy in :issue:`45447`.)
+
 importlib.metadata
 ------------------