Add changelog entry

Author: moradology

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Fix handling of optional properties when using apply on view extension ([#259](https://github.com/stac-utils/pystac/pull/259))
 - Fixed issue with setting None into projection extension fields that are not required breaking validation ([#269](https://github.com/stac-utils/pystac/pull/269))
+- Remove unnecessary `deepcopy` calls in `to_dict` methods to avoid costly overhead ([#273](https://github.com/stac-utils/pystac/pull/273))
+
 
 ### Changed