Issue #9738: PyUnicode_FromFormat() and PyErr_Format() raise an error on
a non-ASCII byte in the format string.

Document also the encoding.
diff --git a/Misc/NEWS b/Misc/NEWS
index 04eb82f..ca64883 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #9738: PyUnicode_FromFormat() and PyErr_Format() raise an error on
+  a non-ASCII byte in the format string.
+
 - Issue #4617: Previously it was illegal to delete a name from the local
   namespace if it occurs as a free variable in a nested block.  This limitation
   of the compiler has been lifted, and a new opcode introduced (DELETE_DEREF).