Swap dependency fastapi to fastapi-slim

[metakot]

So the author of fastapi makes this really unwanted change. See fastapi/fastapi#11522
Some authors (like this) already did the rollback, maybe we too?

[dantownsend]

I think for Piccolo Admin, using fastapi-slim is sufficient, so makes sense to change it.

[metakot]

The horonable sir of FastAPI did the [standard] optional dependencies. Issue resolved :)

[dantownsend]

Cool, thanks for the update.