Remove string type declaration of parameter (#1639)

Fixes #1635

because:

* Covariance allows a child's method to return a more specific type than the return type of its parent's method.
* Contravariance allows a parameter type to be less specific in a child method, than that of its parent.

src/FileSystem/Finder/Iterator/RealPathFilterIterator.php

@@ -77,7 +77,7 @@ public function accept()
      *
      * @return string regexp corresponding to a given string or regexp
      */
-    protected function toRegex(string $str): string
+    protected function toRegex($str): string
     {
         return $this->isRegex($str) ? $str : '/' . preg_quote($str, '/') . '/';
     }