Use Locator type for Get CSS Property Value keyword

Author: b-vamsipunnam

src/SeleniumLibrary/keywords/element.py

@@ -1279,7 +1279,7 @@ def _selenium_keys_has_attr(self, key):
 
     @keyword("Get CSS Property Value")
     def get_css_property_value(
-            self, locator: Union[WebElement, str], css_property: str
+            self, locator: Locator, css_property: str
     ) -> str:
         """Returns the computed value of ``css_property`` from the element ``locator``.