Fix mangled __slots__ attribute lost for subclass instances (#506)

[uttam12331]

Summary

Closes #506.

_dict_from_slots unmangled each slot name using the concrete instance type:

def unmangle(attribute):
    if attribute.startswith('__') and attribute != '__weakref__':
        return '_{type}{attribute}'.format(type=type(object).__name__, attribute=attribute)
    ...

A mangled slot such as __id is stored under the name of the class that declared it (e.g. _Parent__id). For a subclass instance, type(object).__name__ is the subclass, so the lookup used the wrong name (_Child__id), hasattr(...) returned False, and the parent-defined slot was silently dropped:

class A:
    __slots__ = ('__id',)
    def __init__(self, id): self.__id = id

class B(A):
    pass

DeepDiff._dict_from_slots(A(5))   # {'__id': 5}
DeepDiff._dict_from_slots(B(5))   # {}   <-- parent slot lost

This is the root cause behind the unprocessed / dropped-attribute behavior reported when comparing an instance of a class with an instance of its subclass where a slot uses a mangled (dunder-prefixed) attribute — e.g. bson.ObjectId vs a subclass of it.

Fix

Pair each slot with the class in the MRO that declared it, and unmangle using that class's name. Non-mangled slots are unaffected.

Tests

Added test_dict_from_slots_of_subclass_with_mangled_attribute, asserting the parent-declared mangled slot is collected for both the base and the subclass instance. It fails on master ({} for the subclass) and passes with this change. The existing slots tests still pass; flake8 introduces no new warnings; added a CHANGELOG entry.