Avoid psych version constraint in gemspec

[floehopper]

In this commit we avoided a breaking change in psych v4 in ruby v3.1 which was shipped with psych v4 which int turn included a breaking change that was mentioned in the ruby v3.1 release notes (see the "Other notable changes since 3.0" section).

I haven't investigated, but I suspect the code using psych is in rubocop, so we might be able to avoid adding the constraint on the psych version if we update to a more recent version of rubocop...?