[WIP]REGR: Fixed reading from public S3 buckets with credentials
Closes https://github.com/pandas-dev/pandas/issues/34626

This works in 1.0.4 I think, so no whatsnew.

I'd like to wait for https://github.com/pandas-dev/pandas/pull/34266 to get in first. I'll need to update things.

And in the future, I'd like to deprecate this behavior in favor of something like

```python
pd.read_csv("s3://path/to/key", storage_options={"anon": True})
```

It should be easy to detect the cases when we need to do that. We just don't have the `storage_options` kwarg yet.