Add gst-python dependency if python variant used
Add gst-python dependency if python variant used

Closes #30.

shoreflyer/cerbero,multipath-rtp/cerbero,shoreflyer/cerbero,multipath-rtp/cerbero,shoreflyer/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,EricssonResearch/cerbero,EricssonResearch/cerbero,multipath-rtp/cerbero,shoreflyer/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,EricssonResearch/cerbero,shoreflyer/cerbero