fix items and collection download and pypgstac installation

[emileten]

I was exploring a potential issue raised in a discussion and this made me realize the current maxar demo notebook was broken :

• the collection and item download cells were broken. Changed to download from https://github.com/vincentsarago/MAXAR_opendata_to_pgstac using wget.
• the cells inserting the metadata to the DB were broken because we were missing the psycopg dependency. I added it at the top.

[review-notebook-app]

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[vincentsarago]

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