0.65 TypeError: AsyncClient.__init__() got an unexpected keyword argument 'app'

Just stared seeing this error today after the release of `httpx` 0.28.0:

```
File "/opt/hostedtoolcache/Python/3.11.10/x64/lib/python3.11/site-packages/datasette/app.py", line 1601, in request
    async with httpx.AsyncClient(app=self.app) as client:
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: AsyncClient.__init__() got an unexpected keyword argument 'app'
```