Make compatible with 'proper' custom_elements:3.x releases

Discussed Wed 14 August with @fago, who said there's nothing against just dropping the explicit dependency on custom_elements. We depend on it indirectly through lupus_ce_renderer.

If I get to be the person to make the next lupus_decoupled release, I'll merge this. (It's not on my direct timeline ...yet.)

Automatically closed - issue fixed for 2 weeks with no activity.