[ast.Str] Use ast.Constant instead pending removal in python 3.14

src/_pytest/assertion/rewrite.py

@@ -680,7 +680,7 @@ class AssertionRewriter(ast.NodeVisitor):
             if (
                 expect_docstring
                 and isinstance(item, ast.Expr)
-                and isinstance(item.value, ast.Str)
+                and isinstance(item.value, ast.Constant)
             ):
                 doc = item.value.s
                 if self.is_rewrite_disabled(doc):