allow multiple requirement files to be passed in

[infcted]

when trying to pass in multiple requirement files, dh_virtualenv uses the last

override_dh_virtualenv:
    dh_virtualenv \
        --sourcedirectory="opt/myproject/dist" \
        --requirements=core_requirements.txt \
        --requirements=common_requirements.txt \

[infcted]

https://github.com/spotify/dh-virtualenv/pull/302 this seems to of been tried here