Update libmpdec license dates (GH-21216)

Author: Stefan Krah

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