Add gst-python dependency if python variant used

[superdump]

Need to test also on OS X as I guess you tested on Linux. And perhaps it should only be enabled for host builds and not cross-compilation targets.

[zaheerm]

Yes, I only tested on Linux. I used a controlled build with docker with this https://github.com/zaheerm/cerbero/blob/master/Dockerfile to verify it works. I suppose it would be useful for cross-compiled windows targets but a lot less useful for mobile targets.

[superdump]

Actually, you would enable the python variant only for targets where python is interesting, so this is fine. Thanks!