debian/control: add build-dependency `python2` for `dh_python2`
debian/control: add build-dependency `python2` for `dh_python2`

Confusingly, `dh_python3` is provided by `dh-python`, and `dh_python2`
is provided by `python2`.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy