Fix deprecation warnings coming from pQuery library

[MAILPOET-3980]

lib/Util/pQuery/Html5Parser.php

@@ -2,7 +2,9 @@
 
 namespace MailPoet\Util\pQuery;
 
-class Html5Parser extends \pQuery\HtmlParser {
-  /** @var string|\pQuery\DomNode */
-  public $root = 'MailPoet\Util\pQuery\DomNode';
+use MailPoetVendor\pQuery\HtmlParser;
+
+class Html5Parser extends HtmlParser {
+  /** @var string|DomNode */
+  public $root = DomNode::class;
 }