Update `pystac` and `pystac-client` requirements & notebooks for 1.0

[gjoseph92]

As noted in #64, once 1.0 versions of pystac and pystac-client are released, we should:

• Update required versions to 1.0 - Switch examples back to pystac again #81
• Update docs & example notebooks (ItemCollection comes from pystac now, not pystac-client) - Switch examples back to pystac again #81
• Remove PystacClientItemCollection - Support pystac.ItemCollection #64

cc @TomAugspurger

[gjoseph92]

This is done I believe; added links to relevant PRs inline. Note that we don't have a direct dependency any pystac libraries (just pystac-client for the Binder examples), so perhaps this isn't really done. stackstac can function without pystac installed though, so I don't think it should actually be a dependency. Maybe a __version__ check in code though?