Fallback to anonymous access for s3 urls

Opening a public/anonymous-access S3 file with the default fsspec options fails: ```python import asdf af = asdf.open('s3://stpubdata/roman/nexus/soc_simulations/r00340/l1/r0034001001001001001_0001_wfi01_f062_uncal.asdf', lazy_tree=True) ``` with ``` NoCredentialsError: Unable to locate credentials ``` It's possible to work around this by setting an environment variable `FSSPEC_S3='{"anon": true}'` but it would be more user friendly if we had a fallback that retried using `anon=True` (for the fsspec.open call in generic_io) for s3 urls.