Fix unprocessed key error

[SlavaSkvortsov]

If an object contains "unprocessed" string the lib crashes

>>> from deepdiff import DeepHash
>>> DeepHash('unprocessed')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "venv/lib/python3.8/site-packages/deepdiff/deephash.py", line 151, in __init__
    self._hash(obj, parent=parent, parents_ids=frozenset({get_id(obj)}))
  File "venv/lib/python3.8/site-packages/deepdiff/deephash.py", line 410, in _hash
    result, counts = self.hashes[obj]
ValueError: not enough values to unpack (expected 2, got 0)

I understand that later this key is used to return unprocessed objects, but I think this approach is better because you can always grab unprocessed objects using the UNPROCESSED_KEY constant.

Also, I removed RESERVED_DICT_KEYS constant - I think it was useless.

[seperman]

@SlavaSkvortsov Thanks. That makes sense.

[seperman]

Hi @SlavaSkvortsov
DeepDiff 5.6.0 is released and it includes your PR.
Thank you for your contribution!