Request: add type hints or type stubs for `mypy`

Running `mypy==1.6.1` on a module with `rapidjson` imported:

```
module.py:3:1: error: Cannot find implementation or library stub for module named "rapidjson"  [import-not-found]
    import rapidjson
    ^
```

Any chance we can add type hints, or make a type stub for `rapidjson`?