Add PYBUILD_SNAKE to let python from venv and install to venv

[nw-wind]

When the developer uses python venv he may want to specify python interpreter with the full path.
The example of the debian/rules:

DH_ARGS=--with python3 --buildsystem=pybuild
PYBUILD_SNAKE=/opt/my_product_in_venv/
%:
    dh $@ $(DH_ARGS)

This allow install all the code into virtual environment of other debian package. For example the xxx-lib.deb contains just venv and aaa.deb & bbb.deb has the modules of two separate utilities etc.