minor #42151 Use is_iterable() function when possible (javiereguiluz)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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Use is_iterable() function when possible

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

PHP added `is_iterable()` in PHP 7.1.0 (see https://www.php.net/manual/en/function.is-iterable.php) and Symfony 4.4 requires `>=7.1.3`, so maybe we can use this function to simplify some code.

Commits
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6919c1317d Use is_iterable() function when possible

Finder.php

@@ -669,7 +669,7 @@ public function append($iterator)
             $this->iterators[] = $iterator->getIterator();
         } elseif ($iterator instanceof \Iterator) {
             $this->iterators[] = $iterator;
-        } elseif ($iterator instanceof \Traversable || \is_array($iterator)) {
+        } elseif (is_iterable($iterator)) {
             $it = new \ArrayIterator();
             foreach ($iterator as $file) {
                 $file = $file instanceof \SplFileInfo ? $file : new \SplFileInfo($file);