Make sure to infer the arguments to the slice() builtin

Close pylint-dev/pylint#2770

astroid/brain/brain_builtin_inference.py

@@ -509,6 +509,7 @@ def infer_slice(node, context=None):
         raise UseInferenceDefault
 
     infer_func = partial(helpers.safe_infer, context=context)
+    args = [infer_func(arg) for arg in args]
     for arg in args:
         if not arg or arg is util.Uninferable:
             raise UseInferenceDefault