Merge pull request #686 from ruby/lazily-load-stringio

Make to load stringio lazily

lib/psych/nodes/node.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-require 'stringio'
 require_relative '../class_loader'
 require_relative '../scalar_scanner'
 
@@ -56,6 +55,7 @@ def to_ruby(symbolize_names: false, freeze: false, strict_integer: false)
       #
       # See also Psych::Visitors::Emitter
       def yaml io = nil, options = {}
+        require "stringio"
         real_io = io || StringIO.new(''.encode('utf-8'))
 
         Visitors::Emitter.new(real_io, options).accept self