build: bump to 1.3.20 and update dependencies

Raise the dependency floors so a new install cannot pick up a PyJWT
release with a known vulnerability.

- Raise the floors for PyJWT, pydantic, pydantic-settings,
  python-dotenv and validators to their current releases.
- Add the 1.3.20 entry to the changelog.

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 All notable changes to this project will be documented in this file.
 
+## [1.3.20] - 2026-11-04
+
+### Changed
+- Raised the minimum versions of `pydantic`, `pydantic-settings`, `python-dotenv`, `PyJWT` and `validators` to their current releases.
+
+### Security
+- Required `PyJWT` 2.15.1 or later; releases before 2.14.0 have a known vulnerability.
+
 ## [1.3.19] - 2026-09-04
 
 ### Fixed