improve types for metadata and other dict-like types

[xavdid]

Why?

In #1762 we changed StripeObject to no longer be dicts. But, there are many places where we refer to generated map-like types as dicts (like metadata). Those need to be updated to communicate that they're stripe objects at runtime, but not be picky about potential type errors.

Enter the UntypedStripeObject. It's a small manual class that exposes its __getattr__ function (instead of hiding it behind an if TYPE_CHECKING block, like StripeObject does). As a result, it'll never show a type error for key access (just like a normal dict). Note that this exists purely for typecheking, so it has no runtime implications. It's just used to more accurately represent the shape of metadata

For reviewers

The manual change is the new class definition in _stripe_object and its associated tests. Everything else is codegen.

What?

• write UntypedStripeObject
• add type tests
• codegen that into a ton of places
• tweak changelog to be more accurate
• assert_type is available in all supported typing_extension versions now, so move it out of linting jail

See Also

• v15: Type stubs for metadata and Event.data.object don't match runtime types #1782
• ⚠️ StripeObject no longer inherits from dict #1762