Fix PHP 8.1 compatibility

[kylekatarnls]

Fix for:

Deprecation Notice: Return type of PhpOption\Some::getIterator() should either be compatible with
IteratorAggregate::getIterator(): Traversable,
or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in
vendor/phpoption/phpoption/src/PhpOption/Some.php:152

which occur when loading the class in PHP 8.1

[GrahamCampbell]

Thanks for this. I've fixed this by instead actually adding the return type. This was not a BC break because the classes where that needed to happen are final.