python-apt regression

USN-4247-1 fixed vulnerabilities in python-apt. The updated packages caused
a regression when attempting to upgrade to a new Ubuntu release. This
update fixes the problem.

We apologize for the inconvenience.

Original advisory details:

It was discovered that python-apt would still use MD5 hashes to validate
certain downloaded packages. If a remote attacker were able to perform a
machine-in-the-middle attack, this flaw could potentially be used to install
altered packages. (CVE-2019-15795)

It was discovered that python-apt could install packages from untrusted
repositories, contrary to expectations. (CVE-2019-15796)