Add CallableBool.__hash__()

[Reto Aebersold]

As ​CallableBool has no hash method, 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 method solves this problem:

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

[Tim Graham]

It seems a bit odd to have such code, but I suppose it doesn't hurt.