Ruby 2.7 deprecates taint and it no longer has an effect.
The lack of taint support should not cause a problem in
previous Ruby versions.
I'm not sure if the untaint calls in deduplicate are still needed
after the removal of tainting in the parser. If they are not
needed, they should be removed.
https://github.com/ruby/psych/commit/73c1a2b4e0