[rubygems/rubygems] Add missing open3 requires

https://github.com/rubygems/rubygems/commit/06ad654120

lib/rubygems/source/git.rb

@@ -102,6 +102,7 @@ class Gem::Source::Git < Gem::Source
       success = system @git, 'reset', '--quiet', '--hard', rev_parse
 
       if @need_submodules
+        require "open3"
         _, status = Open3.capture2e(@git, 'submodule', 'update', '--quiet', '--init', '--recursive')
 
         success &&= status.success?

test/rubygems/test_gem_ext_cmake_builder.rb

@@ -9,6 +9,8 @@ class TestGemExtCmakeBuilder < Gem::TestCase
     # Details: https://github.com/rubygems/rubygems/issues/1270#issuecomment-177368340
     pend "CmakeBuilder doesn't work on Windows." if Gem.win_platform?
 
+    require "open3"
+
     begin
       _, status = Open3.capture2e('cmake')
       pend 'cmake not present' unless status.success?

test/rubygems/test_gem_source_git.rb

@@ -68,6 +68,7 @@ class TestGemSourceGit < Gem::TestCase
     git_gem 'b'
 
     Dir.chdir 'git/a' do
+      require "open3"
       output, status = Open3.capture2e(@git, 'submodule', '--quiet', 'add', File.expand_path('../b'), 'b')
       assert status.success?, output