USN-4552-3: Pam-python regression

USN-4552-1 and USN-4552-2 fixed a vulnerability in Pam-python. The update introduced a regression which prevented PAM modules written in Python from importing python modules from site-specific directories. We apologize for the inconvenience. Original advisory details: Malte Kraus discovered that Pam-python mishandled certain environment variables. A local attacker could potentially use this vulnerability to execute programs as root.