In Ruby < 3.0, the superclass of StringIO was actually already `Data`,
but it doesn't have the expected shape. So, on these earlier versions it errors:
> NoMethodError: undefined method `members' for #<StringIO:0x00005641dd5f2880>
> vendor/bundle/ruby/2.6.0/gems/psych-5.2.5/lib/psych/visitors/yaml_tree.rb:170:in `visit_Data'
This test doesn't fail on 2.7, presumably because it can pull in a newer `stringio` version.
0f40f56268
