request for consistent datetime intake for Item

[itcarroll]

The datetime argument to pystac.Item expects datetime objects, while the properties argument expects a dictionary with string type for start_datetime and end_datetime when these are required. That inconsistency doesn't seem necessary, and likely leads to extra work handling conversions and time-zone ambiguity.

Why not let the datetime argument handle both cases, and not ask users to mess with the properties?

To elaborate, the code below works, but probably shouldn't. But rathern than setting datetime=None, I'd like to set datetime=tspan and skip setting properties.

import pystac
import pandas as pd

tspan = pd.to_datetime(('2022-01-01T00:00:00 -05:00', '2022-01-31T00:00:00 -05:00'))

c = pystac.Collection(id='Test', description=None, extent=None)
i = pystac.Item(
    id='Test-Item',
    geometry={'type': 'Polygon', 'coordinates': (
        ((1, 0),(1, 1),(0, 1),(0, 0),(1, 0)),
    )},
    bbox=(0, 0, 1, 1),
    datetime=tspan[0],
    properties={'start_datetime': tspan[0].isoformat(), 'end_datetime': tspan[1].isoformat()},
)
c.add_item(i)
c.update_extent_from_items()

Would you be interested in a PR that makes the datetime argument handle one or a two-tuple datetime and sets properties as appropriate? Fair warning ... I'd have to learn typing.

[gadomski]

Similar to #905.