Unify Pokemon model for Rust and Python servers (#2700)

## Motivation and Context
Now we have the feature parity between Rust and Python servers (at least
for the Pokémon service's needs) we can use the same model in both.
Closes https://github.com/awslabs/smithy-rs/issues/1508

## Testing
```bash
$ cd smithy-rs/examples
$ make test # test Rust servers
$ cd python
$ make test # test Python servers
```

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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