the default gems couldn't use ruby2_keywords gem.

We avoid to use the current delegate library under the Ruby 2.7.

lib/delegate/delegate.gemspec

@@ -24,6 +24,5 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
-
-  spec.add_runtime_dependency 'ruby2_keywords'
+  spec.required_ruby_version = '>= 2.7'
 end