Followed up behavior change of set

f360ebb306

spec/ruby/library/set/pretty_print_spec.rb

@@ -1,6 +1,7 @@
 require_relative '../../spec_helper'
 require 'set'
 
+ruby_version_is ""..."3.1" do
   describe "Set#pretty_print" do
     it "passes the 'pretty print' representation of self to the pretty print writer" do
       pp = mock("PrettyPrint")
@@ -15,3 +16,4 @@ describe "Set#pretty_print" do
       set.pretty_print(pp)
     end
   end
+end