fix deps.

[zhouhailin]

prepare-build-system failure,

Package python is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source However the following packages replace it: 2to3 python2-minimal:i386 python2:i386 python2-minimal python2 dh-python python-is-python3

E: Package 'python' has no installation candidate