Add explicit `-> Any` return type annotation to `.pin_read` and `.pin_download`

These functions are currently un-annotated, so some type checkers (e.g. pyright) will try to infer based on the underlying function calls like load_data and by extension function calls like pd.DataFrame.

It would be better if we explicitly annotated these with typing.Any since it would mean the type checker short-circuits this inference and wouldn't raise an error if the user fails to narrow down the type through case-checking from DataFrame | dict etc.

It would also play nicer with extension packages like geopins which extend the interface to give extra return types dynamically, which the type checker doesn't know about.

I understand that pins isn't typically used in the context of heavy-duty type checking but I think this is pretty easy thing to add and would make it more pleasant to work with in VS Code which usually does light pyright analysis via pylance.

I can put in a PR

Locate the definitions of .pin_read and .pin_download, then inspect how their current return types are inferred from calls such as load_data and pd.DataFrame. Add the requested typing.Any return annotations and verify that type checking no longer requires callers to narrow inferred return types.