async client block async loop for a long time

Hi,

I think `stamina.retry_context` should be iterated with `async` , otherwise it will block the loop on connecting error.

see doc of [stamina](https://stamina.hynek.me/en/stable/tutorial.html#async)

```python
async def with_block(code: int) -> httpx.Response:
async for attempt in stamina.retry_context(on=httpx.HTTPError, attempts=3):
with attempt:
async with httpx.AsyncClient() as client:
resp = await client.get(f"https://httpbin.org/status/{code}")
resp.raise_for_status()

return resp
```