TypedDict with extra unspecified fields

[skeggse]

Given that Python does not support expressing a TypedDict that allows extra/unspecified keys, how should one manage dictionaries that have a bunch of keys that aren't relevant to the code that uses them (e.g. come from an external API), and/or where the shape of the dictionary changes relatively frequently by adding new, backwards-compatbile keys (e.g. also from an external API)?

I can imagine using Union[TypeGuardDef, dict[str, Any]], but that makes using values of that type in places that expect only TypeGuardDef/want to access well-defined members of the TypedDict. This isn't a problem in pure Python because we only discover the discrepancy at runtime, which means that there isn't a huge motivation for static typecheckers or Python's core to add this support.

ref #376