[TASK] Add native type declarations in URL (#1163)

Part of #811

Author: oliverklee

CHANGELOG.md

@@ -34,7 +34,7 @@ Please also have a look at our
 - Use more native type declarations and strict mode
   (#641, #772, #774, #778, #804, #841, #873, #875, #891, #922, #923, #933, #958,
   #964, #967, #1000, #1044, #1134, #1136, #1137, #1139, #1140, #1141, #1145,
-  #1162)
+  #1162, #1163)
 - Add visibility to all class/interface constants (#469)
 
 ### Deprecated

src/Value/URL.php

@@ -68,10 +68,7 @@ public function setURL(CSSString $url): void
         $this->url = $url;
     }
 
-    /**
-     * @return CSSString
-     */
-    public function getURL()
+    public function getURL(): CSSString
     {
         return $this->url;
     }