stop using PyAttr everywhere

[Roger-luo]

A confusion has been PyAttr accepting random Python objects but this is actually not legit.

For example, list should use a ListAttr because what it means by compile-time value is slightly different from a runtime object - when at compile-time we can say the hash(list) is returning the object id because it is compile-time constant, but at runtime this is not hashable or should be composing all its elements to perform the expensive hash.