Support Sequence[PystacItem] as an argument to stackstac.stack() method

[philvarner]

The stack method currently supports numerous forms of data for the items parameter:

def stack(
    items: Union[ItemCollectionIsh, ItemIsh],
...

ItemSequence = Sequence[ItemDict]

ItemIsh = Union[SatstacItem, PystacItem, ItemDict]
ItemCollectionIsh = Union[
    SatstacItemCollection, PystacCatalog, PystacItemCollection, ItemSequence
]

However, one thing it doesn't support is a Sequence[PystacItem]. This would allow clean integration with pystac-client:

stack = stackstac.stack(list(Client(...).search().items()))

(also the get_all_items() method in the docs is now deprecated in favor of the items() iterator)

This change would also require a change to:

def items_to_plain(items: Union[ItemCollectionIsh, ItemIsh]) -> ItemSequence:

to handle the case where the Sequence is either a dict representing an item or a pystac.Item.

[philvarner]

🎉