devel/py-Jinja2: Update to 3.1.6 (security release)

• Set the PYTHONPATH variable to find the Jinja2 module required by tests without installing it in the system.

Release changes: https://jinja.palletsprojects.com/en/stable/changes/#version-3-1-6
Reported by: pallets/jinja <jinja@noreply.github.com>

Security: CVE-2025-27516