Stumbled upon this, works quite well, could be something to do in Pixelmator Pro since it's already have ML-stuff?
edit: click API up in the right corner and make it happen!
What features would you like to see in Pixelmator Pro?
Totally agree.......remove.bg would be a GREAT feature to have in Pixelmator Pro. Placing a different background in Pixelmator is a hassle.