Add Type hints to function.py

[Caiofcas]

Continuation of #1821 (issue #1533 ).

Notable changes:

• Add _JsonSafeTypes in utils.py in an attempt to properly catch all cases in .to_dict() methods.
• Same Mapping -> MutableMapping and .copy() -> deepcopy() changes in SF() function as was done in Q().
• Still having some Any types in kwargs and params, maybe create a new ticket to tighten these up after initial type hints are done? don't know if I can just go by the test cases and assume types not present in them are invalid, the way the functions are written it seems purposefully loose.

[cla-checker-service]

💚 CLA has been signed

[miguelgrinberg]

Add _JsonSafeTypes in utils.py in an attempt to properly catch all cases in .to_dict() methods.

Does this type need the _prefix?

Same Mapping -> MutableMapping and .copy() -> deepcopy() changes in SF() function as was done in Q().

Looks good.

Still having some Any types in kwargs and params, maybe create a new ticket to tighten these up after initial type hints are done? don't know if I can just go by the test cases and assume types not present in them are invalid, the way the functions are written it seems purposefully loose.

Yes, this will have to be treated as a separate change. You are correct, this was designed many years ago with the idea of being completely dynamic. I think in the end what I'm going to end up doing is add the kwargs explicitly in all of these, but let's not worry about that for this effort.

[Caiofcas]

Does this type need the _prefix?

It's not required, I put it there from the perspective of it's an internal library helper type and should probably not be used from the outside. Similar to a numpy _ArrayType or other internal types like it.

[miguelgrinberg]

@Caiofcas Merging this change, but note that I have renamed _JSONSafeTypes to JSONType. There is a bunch of other internal things in utils.py (and many more in other modules) that do not use the _ prefix, so to me it seems more consistent to not use it.

But anyway, you are making great progress! :)