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