Merge branch 'master' of github.com:Payum/Payum into merge-up/1.7.x-master

* 'master' of github.com:Payum/Payum: (42 commits)
  Update README.md
  Create FUNDING.yml
  Update get-it-started.md (forgot about namespace)
  Added PayU India to supported gateway list.
  Add headers property
  Add Blackfire as a sponsor
  Fix subtree split on GH Actions (#915)
  Add use statements for all classes
  Replace string class names with class constant
  Remove un-used imports
  Add additional CS rules
  Add Yoda style fixer
  Add Array CS set
  Add CleanCode rule
  Remove un-used imports
  Add PHPUnit CS rule
  Add Docblock and Comments CS rules
  Add Spaces CS rule
  Run ECS on GH Actions
  Add PSR-12 coding standard and fix CS
  ...

Author: pierredup

Model/Identity.php

@@ -36,10 +36,7 @@ public function __unserialize(array $data)
         list($this->id, $this->class) = $data;
     }
 
-    /**
-     * @return string
-     */
-    public function __toString()
+    public function __toString(): string
     {
         return $this->class . '#' . $this->id;
     }