[#478] FIX: link condition element_is_selected to selenium is_selected()

adjusting the `element_is_selected` condition so that it calls the
Selenium element's `is_selected()` method. instead of looking for
the attribute `elementIsSelected`.

Closes #478

selene/core/match.py

@@ -243,7 +243,9 @@ def values_containing(
     )
 
 
-element_is_selected: Condition[Element] = element_has_attribute('elementIsSelected')
+element_is_selected: Condition[Element] = ElementCondition.raise_if_not(
+    'is selected', lambda element: element().is_selected()
+)
 
 
 def element_has_value(expected: str) -> Condition[Element]: