USN-4975-2: Django vulnerability

USN-4975-1 fixed a vulnerability in Django. This update provides the corresponding update for Ubuntu 16.04 ESM. Original advisory details: Rasmus Lerchedahl Petersen and Rasmus Wriedt Larsen discovered that Django incorrectly handled path sanitation in admindocs. A remote attacker could possibly use this issue to determine the existence of arbitrary files and in certain configurations obtain their contents. (CVE-2021-33203)