Bumped copyright for 2021.

LICENSE.txt

@@ -1,5 +1,5 @@
 
-   Copyright (c) 2001-2020 Ryan C. Gordon and others.
+   Copyright (c) 2001-2021 Ryan C. Gordon and others.
 
    This software is provided 'as-is', without any express or implied warranty.
    In no event will the authors be held liable for any damages arising from