make compatible with twig 3.9, twig_get_attribute is now internal func (#441)

Author: jtallant

src/Node/GetAttrNode.php

@@ -10,6 +10,7 @@
 use Twig\Compiler;
 use Twig\Environment;
 use Twig\Error\RuntimeError;
+use Twig\Extension\CoreExtension;
 use Twig\Extension\SandboxExtension;
 use Twig\Node\Expression\GetAttrExpression;
 use Twig\Node\Node;
@@ -144,6 +145,12 @@ public static function attribute(
             return $object->$item;
         }
 
+        // Note: Since twig:3.9 the 'twig_get_attribute' function was renamed to CoreExtension::getAttribute.
+        //       Because this is an internal function of twig, the authors could break it in a minor version.
+        if (!function_exists('twig_get_attribute')) {
+            return CoreExtension::getAttribute($env, $source, $object, $item, $arguments, $type, $isDefinedTest, $ignoreStrictCheck);
+        }
+
         return \twig_get_attribute(
             $env,
             $source,