Add @immutable decorator

[sobolevn]

I hope that some generic solution for @immutable type decorator can be created.

What I need? I need to make mypy treat some types as immutable (they can still be mutable at runtime).
Example:

@immutable
class User(object):
    def __init__(self, name: str) -> None:
         self.name = name  # ok

user = User('example')  # ok 
user.name = 'other'  # mypy error

Why? Because I want to be able to express this idea without actually modifing how things work in real code.