Add conversion support for the jiff datetime library

[rumpelsepp]

The author of ripgrep has started a datetime library called jiff. The author provides a in depth comparison with other datetime libraries here. I figure, jiff might gain popularity quickly and it could be a good improvement for pyo3 to provide an implementation for converting to python datetime objects (as it is available for, e.g., chrono).

[bschoenmaeckers]

Sounds like a great exercise. I maybe take a stab at this next week.