[python] fix: Annotate free-form object as dict in Python

[robertschweizer]

Model properties annotated only with type: object are currently turned into Union[str, Any] in Python. This is wrong, since they must be dictionaries. They cannot be strings.

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@multani @fa0311 @wing328

[wing328]

LGTM. Let's give it a try.

[mtraynham]

@robertschweizer What if the free form object is a dictionary or array? This change kind of prevents you from being able to parse it yourself from a string.

[robertschweizer]

A "free form object" must have type: object in the JSON schema I believe. So it must be a "dictionary" in Python terms, it cannot be a string or array.