- Compatibility with older Python versions
- use `Optional` and `Union` instead of `... | None` and `a | b`
- use `typing_extensions.Protocol` instead of `typing.Protocol`
- use `typing.Dict`, `typing.List`, etc. instead of the concrete types.
- Fix backend `.get()` annotations; not all were marked as `Optional[str]`
- Don't return anything from `Backend.set()` methods.
- The `Coder.decode_as_type()` type parameter must be a type to be
compatible with `ModelField(..., type_=...)`.
- Clean up `Optional[]` use, remove where it is not needed.
- Clean up variable use in decorator, keeping the raw cached value
separate from the return value from the wrapped endpoint.
- Annotate the wrapper as returning either the original type _or_ a
Response (returning a 304 Not Modified response).
- Clean up small edge-case where `response` could be `None`.
- Correct type annotation on `JsonCoder.decode()` to match `Coder.decode()`.
