Use Stringable interface as much as possible

Author: VincentLanglet

Transport/TransportInterface.php

@@ -18,14 +18,12 @@
 /**
  * @author Fabien Potencier <fabien@symfony.com>
  */
-interface TransportInterface
+interface TransportInterface extends \Stringable
 {
     /**
      * @throws TransportExceptionInterface
      */
     public function send(MessageInterface $message): ?SentMessage;
 
     public function supports(MessageInterface $message): bool;
-
-    public function __toString(): string;
 }