chore(main): release 0.3.21

Author: a2a-bot

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Changelog
 
+## [0.3.21](https://github.com/a2aproject/a2a-python/compare/v0.3.20...v0.3.21) (2025-12-12)
+
+
+### Features
+
+* Implement Agent Card Signing and Verification per Spec ([#581](https://github.com/a2aproject/a2a-python/issues/581)) ([8a76730](https://github.com/a2aproject/a2a-python/commit/8a767305d0a6ecd8bbca4ede643e64ecba01edee))
+
+
+### Documentation
+
+* Fixing typos ([#586](https://github.com/a2aproject/a2a-python/issues/586)) ([5fea21f](https://github.com/a2aproject/a2a-python/commit/5fea21fb34ecea55e588eb10139b5d47020a76cb))
+
 ## [0.3.20](https://github.com/a2aproject/a2a-python/compare/v0.3.19...v0.3.20) (2025-12-03)