Missing Hash function in CallableBool

[Reto Aebersold]

As ​CallableBool has no hash function, membership test operations for sets do not work:

from django.utils.deprecation import CallableTrue
assert CallableTrue in {True}

TypeError: unhashable type: 'CallableBool'

Adding a hash function solves this problem:

def __hash__(self):
    return hash(self.value)

I'm happy to create a PR if this make sense.