… warnings

As of PHP 8.1, PHP started throwing deprecation warnings along the lines of:
```
Deprecated:  Return type of [CLASS]::[METHOD]() should either be compatible with [PHP NATIVE INTERFACE]::[METHOD](): [TYPE], or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

These type of deprecation notices relate to the [Return types for internal methods RFC](https://wiki.php.net/rfc/internal_method_return_types) in PHP 8.1.

Basically, as of PHP 8.1, these methods in classes which implement PHP native interfaces are expected to have a return type declared.
The return type can be the same as used in PHP itself or a more specific type. This complies with the Liskov principle of covariance, which allows the return type of a child overloaded method to be more specific than that of the parent.

As this package still has a minimum PHP requirement of PHP 5.6, the return type (PHP 7.0 feature) can not be added, so I've added the attribute instead.

While attributes are a PHP 8.0 feature only, due to the syntax choice for `#[]`, they will ignored in PHP < 8 and can be safely added.

Note: these deprecations were visible in the test summary, but the tests - as they currently are - are not set up correctly to fail builds on deprecation notices, which is probably why this went unnoticed.