feat: Add python picker

[jnumainville]

Fixes #294
Creates python side of picker component along with section and items.

Props that are not currently supported:
items - we don't support passing in a callback as a child and we support passing in lists in children, so this will not be supported
validate - is a callback we don't support
errorMessage - specifically a callback
onLoadMore - irrelevant due to supporting a table source

[mofojed]

We can totally wire up the onOpenChange, onFocusChange, onLoadMore callbacks. Note the onLoadMore we should have a note that it's irrelevant when using a table as a data source, but no technical reasons why we can't wire these up.

[dsmmcken]

I would just skip onloadmore, not worth it, table source is strictly better.

[jnumainville]

Ok, I wasn't sure of what I should list here in terms of if they must be supported now or not but I removed onOpenChange and onFocusChange as they will be supported.

[mofojed]

We can be a little more specific here. In the React typings, the keys allowed are: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/eaba7f4fb1a43d69a4e06084e530db36da0b63cd/types/react/index.d.ts#L233
Key could just be the same as Stringable and we should be fine (str, number, bool types). @bmingles also has a comment about how boolean is supported as a key on his PR.
We certainly do not want to allow non-serializable objects to be passed in as the key, so Any is not really appropriate.

[bmingles]

I should clarify that React's Key type is restricted to number | string only. Spectrum's Picker is just incorrectly declaring that as the type for selection keys.

For this particular case, we should be able to use Stringable so that we get support for bool included, but there are likely other cases where the Key type would actually be correct, so the type may still be useful. In such case it should probably be defined as Key = Union[str, int, float]