chore: Export intended public imports

It looks like MyPy and Pyright currently require imports like these to
be re-exported in order to be used. Doing this raises a lint error for
me due to Pyright:

```python
import aw_client

aw_client.ActivityWatchClient()  # "ActivityWatchClient" is not exported from module "aw_client"
```

Ref: https://github.com/microsoft/pyright/blob/main/docs/typed-libraries.md#library-interface
Ref: microsoft/pyright#3409
Ref: python/mypy#8754 (comment)

aw_client/__init__.py

@@ -1 +1,3 @@
-from .client import ActivityWatchClient  # noqa
+from .client import ActivityWatchClient
+
+__all__ = ("ActivityWatchClient",)