Actually this is possible by searching for UIApplication up in the responder chain and invoking openURL
on it, like described in this response : https://stackoverflow.com/a/28037297/554203.
This is the code that works for XCode 8, Swift 3.0 and iOS10 (again, extracted from Julio Bailon's response above):
let url = NSURL(string:urlString)
let context = NSExtensionContext()
context.open(url! as URL, completionHandler: nil)
var responder = self as UIResponder?
while (responder != nil){
if responder?.responds(to: Selector("openURL:")) == true{
responder?.perform(Selector("openURL:"), with: url)
}
responder = responder!.next
}