遇到了同样的问题。原因应该是ubuntu20.10完全丢弃了python2仓库。在Ubuntu 20.04上运行apt list python-apt:
$apt list python-apt
python-apt/focal 2.0.0 amd64
python-apt/focal 2.0.0 i386
在Ubuntu 20.10上:
$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