Feature/merge 3 0 develop (#298)

* merging 3.0 into develop

* N°5102 - Allow to send emails (eg. notifications) using GSuite SMTP and OAuth
* migration to iTop 3.1

Co-authored-by: Eric Espie <eric.espie@combodo.com>

lib/guzzlehttp/promises/src/TaskQueueInterface.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace GuzzleHttp\Promise;
+
+interface TaskQueueInterface
+{
+    /**
+     * Returns true if the queue is empty.
+     *
+     * @return bool
+     */
+    public function isEmpty();
+
+    /**
+     * Adds a task to the queue that will be executed the next time run is
+     * called.
+     */
+    public function add(callable $task);
+
+    /**
+     * Execute all of the pending task in the queue.
+     */
+    public function run();
+}