Type error mixing single and array children in tsx using class based widgets

[agubler]

Bug

Mixing single and array children in tsx with class based widget causes a typing error.

class App extends WidgetBase {
	render() {
		return (
			<div>{this.children}</div>
		);
	}
}
const domNode = document.createElement('div');
const r = renderer(() => (
	<App>
		// causes a typing issue as mixing arrays and 
		// single items is not supported
		{[<div>first</div>, <div>second</div>]}
		{<div>third</div>}
	</App>
));