Getting TypeError for vanilla ECSCluster/FargateCluster example

I'm trying to create an AWS cluster for using Dask temporarily. In former times there was some "dask-ec2", but that is no more, so I thought dask-cloudprovider is the new way to go. I've created fresh AWS access key ID and secret and ran the vanilla exmple on https://cloudprovider.dask.org/en/latest/.

**What happened**:

```
[…]
/srv/conda/envs/notebook/lib/python3.7/site-packages/aiobotocore/credentials.py in load_credentials(self)
785 for provider in self.providers:
786 logger.debug("Looking for credentials via: %s", provider.METHOD)
--> 787 creds = await provider.load()
788 if creds is not None:
789 return creds

TypeError: object NoneType can't be used in 'await' expression
```

**What I expected to happen**:

I don't expect this to work, but I'm getting the error I don't know how to deal with. So I guess I had expected some other error. ;)

**Minimal Complete Verifiable Example**:

```python
from dask_cloudprovider import ECSCluster, FargateCluster

cluster = FargateCluster( # or ECSCluster
aws_access_key_id="******",
aws_secret_access_key="******",
region_name="******"
)
```

**Environment**:

Linux, Python 3.7.8, plus (pip-installed):

```
$ pip list | egrep awscli\|boto\|dask
aiobotocore 1.0.7
awscli 1.18.120
boto3 1.14.43
botocore 1.15.32
dask 2.23.0
dask-cloudprovider 0.4.0```