Remove final from Observer and Subscriber

lib/Observer.php

@@ -12,7 +12,7 @@ namespace Amp;
  * }
  * $result = $observer->getResult();
  */
-final class Observer {
+class Observer {
     /**
      * @var \Amp\Subscriber
      */

lib/Subscriber.php

@@ -7,7 +7,7 @@ use Interop\Async\Awaitable;
 /**
  * Subscriber implementation returned from implementors of \Amp\Observable.
  */
-final class Subscriber implements Awaitable {
+class Subscriber implements Awaitable {
     /**
      * @var string
      */