Use Constant.value, not Constant.n

Since Python 3.8, class ast.Constant is used for all constants.
Old classes ast.Num, ast.Str, ast.Bytes, ast.NameConstant and
ast.Ellipsis are still available, but they will be removed in
future Python releases.

Constants have values, but not always ns.

See sqlalchemy#287 (comment)
See python/cpython#9445
Fixes Pylons/pyramid_mako#47

mako/_ast_util.py

@@ -682,7 +682,7 @@ def visit_Num(self, node):
 
     # newly needed in Python 3.8
     def visit_Constant(self, node):
-        self.write(repr(node.n))
+        self.write(repr(node.value))
 
     def visit_Tuple(self, node):
         self.write('(')