restore script needs updating to python 3

[redshiftzero]

Description

securedrop/install_files/ansible-base/roles/restore/files/restore.py needs updating to python 3

[rmol]

There are other shebang lines that should be updated, among them of course backup.py. They could all be changed to use /opt/venvs/securedrop-app-code, which would ensure the virtualenv never has to be activated in production to avoid confusing failures, but that would pose a problem for developers invoking them while using custom virtualenvs. I think making production use less involved should take priority; if devs need to run things without using the Makefile targets or the dev container, they can specify their python on the command line.