Change divide_into_two to return a tuple instead of a list

[google-labs-jules]

Modified the divide_into_two method of the BitVector class to return a tuple containing the two halves of the split bit vector rather than a list. Updated the return type annotation and docstring. Also updated an unpacking assignment in examples/demo.py.

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PR created automatically by Jules for task 13067570866386383465 started by @schwehr

[google-labs-jules]

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