[py] fix return type hints for screenshot_as_png (#10626)

Author: symonk

py/selenium/webdriver/remote/webelement.py

@@ -660,7 +660,7 @@ def screenshot_as_base64(self) -> str:
         return self._execute(Command.ELEMENT_SCREENSHOT)['value']
 
     @property
-    def screenshot_as_png(self) -> str:
+    def screenshot_as_png(self) -> bytes:
         """
         Gets the screenshot of the current element as a binary data.