debian/control: add missing build-dependency on dh-python
debian/control: add missing build-dependency on dh-python

Fixes warning:

 W: dh_python2:479: Please add dh-python package to Build-Depends

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