I was going to say you can use an Edit Properties LOP, drag the "animationSource" attribute from the "Schemas/SkelBindingAPI" folder on the "From USD" tab in the far left panel of the Edit Parameter Interface dialog... But it seems that this doesn't apply the API schema. Adding the "geomBindTransform" primvar applies the API schema, so there may be something wrong with how Relationships are brought in from that pane? In the meantime, you can edit the "skel:animationSource" parameter that gets created by adding the "usdapischema" tag with a value of "SkelBindingAPI". Then when you set the animationSource parameter, it will set the relationship and apply the SkelBindingAPI schema. Basically what your python code above is doing.
I'll look into the bug about the SkelBindingAPI schema not being applied automatically.