Support Python 3.7 timezone in datetime bindings

[pganssle]

As part of #200 (which added the Python datetime bindings), the update for the full Python 3.7 interface, which includes:

• PyDateTime_TimeZone_UTC
• PyTimeZone_FromOffset
• PyTimeZone_FromOffsetAndName

The timezone API was added to CPython in PR 5032.

I believe this can be accompanied on the Rust side by a PyFixedTimeZone object or something. It's worth noting that no _Check methods accompany this, because you are really not intended to subclass these things.

[pickfire]

Can I get some pointers on how to provide a wrapper for PyDateTime_TimeZone_UTC?

I see that we can do PyObject::from_borrowed_ptr(py, PyDateTimeAPI.TimeZone_UTC) but should we do it the same way as python::NotImplemented()? I wonder how should we handle the singleton in this case?

Also, how can one do <PyXXXUTC as PyTryFrom>::try_from(ob) when implementing FromPyObject for chrono::Utc? I don't get how do we PyTryFrom a thing that does not have a thin wrapper, even if we have, I am still confused if we can use pyobject_native_type! for this?

Context https://github.com/chronotope/chrono/pull/542/files#diff-70ff3919bf785b8ecc0ce5229161d04c1525d549664aa48e30d3415feffb2eb3R125

[davidhewitt]

@pickfire I'm thinking about the correct design for this; please allow me a few days.

[pickfire]

Just discussing, I will only get to use FromOffset (which correlates to chrono FixedOffset) and UTC (which correlates to chrono Utc), so minimally I require those two, but while doing that I can probably add the FixedOffsetWithName as well.

I believe I can just start with PyTimeZone_FromOffset since that is straightforward. Maybe I will keep UTC for later.

[davidhewitt]

@pickfire maybe #1573 gets you started? With that PR you could do ffi::PyDateTime_TimeZone_UTC.as_ref(py) to get a &PyAny representing the singleton.