I apologize as this is perhaps not the appropriate bug tracker to report the issue, but you've probably better hints than me on where the responsibility lies
Recently my "pip install cffconvert" caused ruamel.yaml.clib==0.2.9 to be installed, and the combination of both fails.
With CITATION.cff being https://github.com/OSGeo/gdal/blob/master/CITATION.cff, "cffconvert --validate" fails with a "TypeError: a string or stream input is required". Downgrading to ruamel.yaml.clib==0.2.8 fixes the issue.
I apologize as this is perhaps not the appropriate bug tracker to report the issue, but you've probably better hints than me on where the responsibility lies
Recently my "pip install cffconvert" caused ruamel.yaml.clib==0.2.9 to be installed, and the combination of both fails. With CITATION.cff being https://github.com/OSGeo/gdal/blob/master/CITATION.cff, "cffconvert --validate" fails with a "TypeError: a string or stream input is required". Downgrading to ruamel.yaml.clib==0.2.8 fixes the issue.