Use Psycopg3 instead of Psycopg2

[meksor]

Adresses #20 and adjusts the psycopg2 dependency.
Using the binary package is no longer discouraged, so this is in use because its easiest and does not require system dependencies. Read more here.

Also replaces postgresql:// platform dsn prefixes with the right ones to keep it compatible with old platforms.toml files.

[danielhuppmann]

Doesn't a dsn postgresql+psycopg make it quite long and cumbersome going forward? Wouldn't it make more sense to use postgresql+psycopg2 for any "old" database instances and simply use postgresql for current ones?

[meksor]

That is not our choice, but sqlalchemy's. We could inspect and transform every dsn from our own format so-to-say but i would suspect that is more cumbersome than just using sqlalchemy's way.

[danielhuppmann]

Thanks for the quick fix!