No package matching 'python-apt' is available
Ran into the same problem.
The cause should be ubuntu20.10 totally discards python2 repository.
Run apt list python-apt on Ubuntu 20.04:
$apt list python-apt
python-apt/focal 2.0.0 amd64
python-apt/focal 2.0.0 i386
on Ubuntu 20.10:
$apt show python-apt
Package: python-apt
State: not a real package (virtual)
N: Can't select candidate version from package python-apt as it has no candidate
N: Can't select versions from package 'python-apt' as it is purely virtual
N: No packages found