Python computations to be interoperable with Python's datetime and timedelta objects

[ChristopherRabotin]

PyO3 provides that support for chrono. I don't think that PyO3 would support it for hifitime because that would introduce cyclical dependencies. Hence, the work in https://github.com/PyO3/pyo3/blob/249c0209fd41336b9e68bae14290e598fcad34ea/src/conversions/chrono.rs should be duplicated in hifitime directly.