Phantom types for mutable base types

[sobolevn]

As discussed in #127 (comment) I am openning a new issue.

This code is very risky one to write:

from django.db import models

class  User(models.Model):
    is_paid = models.BooleanField(default=False)

def is_paid_user(value: object) -> bool:
    return isinstance(value, User) and value.is_paid

class PaidUser(User, Phantom, predicate=is_paid_user):
    ...  # metaclass conflict

Why? Because User is mutable. At anytime you can just user.is_paid = True | False to change its PaidUser instance check without mypy noticing the change.

We need to document this problem.

[sobolevn]

Related: wemake-services/mypy-extras#103