update changelogs

Author: dsinghvi

fern/products/sdks/generators/python/changelog/2026-04-20.mdx

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+## 5.5.3
+**`(chore):`** Bump generated `pytest` dev dependency from `^8.2.0` to `^9.0.3` for SDKs
+targeting Python 3.9+. This addresses CVE-2025-71176 (GHSA-6w46-j5rx-g56g),
+a moderate-severity issue with insecure `/tmp/pytest-of-{user}` handling
+on UNIX. Projects that still support Python 3.8 continue to use `pytest ^7.4.0`
+since pytest 9 requires Python 3.9+.
+
+