#6844: do not emit DeprecationWarnings on access if Exception.message has been set by the user.

This works by always setting it in __dict__, except when it's implicitly set in __init__.
diff --git a/Misc/NEWS b/Misc/NEWS
index 555d96e..72aa48e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #6844: Do not emit DeprecationWarnings when accessing a "message"
+  attribute on exceptions that was set explicitly.
+
 - Issue #6846: Fix bug where bytearray.pop() returns negative integers.
 
 - classmethod no longer checks if its argument is callable.