Update libmpdec license dates (GH-21216)

Stefan Krah · web-flow · commit a3ad95dd2176 · 2020-06-29T14:03:01.000+02:00
diff --git a/Doc/license.rst b/Doc/license.rst
@@ -889,7 +889,7 @@ libmpdec
 The :mod:`_decimal` module is built using an included copy of the libmpdec
 library unless the build is configured ``--with-system-libmpdec``::
 
-   Copyright (c) 2008-2016 Stefan Krah. All rights reserved.
+   Copyright (c) 2008-2020 Stefan Krah. All rights reserved.
 
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
