CI: troubleshoot s3fs failures
xref #28612.  Recent failures used 0.3.4, the last passing build I see used 0.2.0
Won't be able to take a look today. Feel free to merge whenever.

cc @martindurant. Thanks @jbrockmendel  Locally, with pandas 0.25.0, s3fs master and fsspec master, a dataframe round-trips to S3 just fine.