Always add dh-python as Build-Depends

[WilliamDEdwards]

Currently, dh-python is not added to Build-Depends. However, as far as I can see, either dh_python3 or dh_python2 is always used.

[kputyra]

I confirm that this is a problem. At first stdeb was ending with the error

dh: error: unable to load addon python3: Can't locate Debian/Debhelper/Sequence/python3.pm in @INC (you may need to install the Debian::Debhelper::Sequence::python3 module)

but apt install dh-python has solved it.