After deserializing from serialized complex objects containing a dictionary (such as MultiPack that uses '_inverse_pack_ref') the KEY of the dictionary becomes string (from previous int for example)

[J007X]

Describe the bug
After deserializing from serialized complex objects containing a dictionary (such as MultiPack that uses '_inverse_pack_ref') the KEY of the dictionary becomes string (from previous int for example). For example in MultiPack. the dict '_inverse_pack_ref'
should be [int, int] but after deserialization (by JSONPickle) it becomes [str, int] and any attempt to using an int (such as pack_id) as key to lookup in such dictionary will generate an KeyError exception instead of the correct result, so currently this can only be workaround as the following addtional string conversion and catch additional errors (which is not ideal):

def get_pack_index(self, pack_id: int) -> int:
    try:
        return self._inverse_pack_ref[pack_id]
    except KeyError:  # sometimes after deserialization the dict have string as key
        s_dict: dict = self._inverse_pack_ref
        s_key = str(pack_id)
        try:
            return s_dict[s_key]
        except KeyError as ke:  # really not found
            raise ProcessExecutionException(
                f"Pack {pack_id} is not in this multi-pack."
            ) from ke

To Reproduce
the 2 test cases of MultiEntryStructure in entry_data_structures_test.py can be used.
Basically serialize a MultiPack with at least 2 data_packs in it to a string , then deserialize from the string, and re-link the data_packs. This will generate an exception with the new changes that refer/accessing the data_packs in a multi-pack by pack_id instead of by internal array index (#559 related enhancements). So an old version of "get_pack_index" in multi-pack not handling the int -> str change of the dictionary will fail to lookup for the pack correctly.

Expected behavior
Such dictionary will be unchanged after deserialization

Screenshots

Environment (please complete the following information):
(non-specific)

Additional context
Per discussion with Hector, this seems to relate to the following discussion (when serialized to JSON):
https://stackoverflow.com/questions/1450957/pythons-json-module-converts-int-dictionary-keys-to-strings