Support casting of datetime with timezone information

[lbonn]

Updated (and fixed) version of #1144.

Before these changes, nanobind would silently accept datetime objects with timezone information and treat them as local time.

In the naive timestamp case (tzinfo=None), the cost should be small.

[wjakob]

cc @oremanj

Also needs a changelog explaining how it changes the behavior of the existing type caster. My understanding is that this fixes basically buggy/nonintuitive behavior and so could also be merged as part of a minor version bump. Is that correct?

[lbonn]

Updated the changelog and fixed a memory leak in the limited api implementation.

Indeed, the previous behavior was documented but I don't think there was any case where it is actually desirable so it can probably be included in a minor version bump.