Tiny tweak: Extend copyright notice to 2023 (#1528)

Author: JackKelly

LICENSE.txt

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2015-2022 Zarr Developers <https://github.com/zarr-developers>
+Copyright (c) 2015-2023 Zarr Developers <https://github.com/zarr-developers>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal