plugin: Name differently between python2 and python3
plugin: Name differently between python2 and python3

Those are 2 different binaries and thus should have different
.so names. Just use the $PYTHON_SO for that to happen.

https://bugzilla.gnome.org/show_bug.cgi?id=738157

GStreamer/gst-python,pexip/gst-python,pexip/gst-python,GStreamer/gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,GStreamer/gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,freedesktop-unofficial-mirror/gstreamer__gst-python,pexip/gst-python