Update Python CHANGELOG for kastore version update

Updated CHANGELOG to include bugfixes and new features.

Author: jeromekelleher

python/CHANGELOG.rst

@@ -18,6 +18,11 @@
 - TreeSequences and TableCollections now have a ``metadata_size`` property,
   returning their size in bytes. (:user:`petrelharp`, :pr:`3475`)
 
+**Bugfixes**
+
+- Update to kastore C version 2.1.3 to include file security fix (:user:`jeromekelleher`, 
+  :pr:`3478`).
+
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 [1.0.3] - 2026-05-14
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