Bump version to 4.0.0

Breaking changes already in the codebase (httpx migration, dropping
Python 3.9) warrant a major version bump per semver.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottlovegrove

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "todoist_api_python"
-version = "3.3.0"
+version = "4.0.0"
 description = "Official Python SDK for the Todoist API."
 authors = [{ name = "Doist Developers", email = "dev@doist.com" }]
 requires-python = ">=3.10,<4"