chore: bump version to 0.4.0 and add changelog entry

Author: gregsvo

CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ---
 
-## [Unreleased]
+## [0.4.0] - 2026-08-12
 
 ### Added
 

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "postmark-python"
-version = "0.3.7"
+version = "0.4.0"
 description = "The Official Postmark Python SDK."
 authors = [
     { name = "Greg Svoboda", email = "gsvoboda@activecampaign.com" },