Workaround poor pystac performance, or switch examples away from pystac

[gjoseph92]

Because of stac-utils/pystac#546, using pystac with stackstac is incredibly (nearly unusably) slow.

Either:

• Use some of the workarounds mentioned in that issue Slowness in Item.to_dict(), seemingly from links stac-utils/pystac#546 (comment)
• Switch all examples away from pystac until this is fixed, and raise a performance warning when passing a pystac collection
• Remove pystac support entirely for now; require users to convert to a dict themselves

[gjoseph92]

Fixed in #81. However, we still need to upgrade to planetary-computer = ^0.4.3 to include microsoft/planetary-computer-sdk-for-python#30