graphql-server v3.0.0b1 does not work with aiohttp in async mode with graphiql

I had to change graphqlview.py at line 155 as  follows

```python
exec_res = (
    [ex if ex is None or isinstance(ex, ExecutionResult) else await ex for ex in execution_results]
    if self.enable_async
    else execution_results
)
```

**from**

```python
exec_res = (
    [await ex for ex in execution_results]
    if self.enable_async
    else execution_results
)
```

