An iterable-style DataPipe for STAC items! Uses pystac for reading the files or URLs. Included a doctest that checks the metadata within the pystac.item.Item object. Added a new section in the API docs and an intersphinx mapping.

Ensure that zen3geo works even when `pystac` is not installed and add `pystac` to the spatial section of the extras dependencies in pyproject.toml.

New poetry 1.2.0 resolver and other optional dependencies like adlfs and contextily!

Decided that since the returned object is a `pystac.Item`, it should probably be `read_to_pystac_item`.

Ensure that a JSON STAC item can be read into a pystac.Item object that contains various spatiotemporal metadata.

Use pytest.importorskip to skip running the doctest when pystac cannot be imported.