Lazily load open3

Since it's a default gem now, it's better to load it as late as
possible.

lib/bundler/source/git/git_proxy.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "open3"
 require "shellwords"
 
 module Bundler
@@ -243,12 +242,14 @@ def allowed_in_path
         end
 
         def capture_and_filter_stderr(uri, cmd)
+          require "open3"
           return_value, captured_err, status = Open3.capture3(cmd)
           Bundler.ui.warn URICredentialsFilter.credential_filtered_string(captured_err, uri) if uri && !captured_err.empty?
           [return_value, status]
         end
 
         def capture_and_ignore_stderr(cmd)
+          require "open3"
           return_value, _, status = Open3.capture3(cmd)
           [return_value, status]
         end

spec/support/helpers.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "open3"
-
 require_relative "command_execution"
 require_relative "the_bundle"
 
@@ -211,6 +209,7 @@ def sys_exec(cmd, env = {})
 
       env = env.map {|k, v| [k.to_s, v.to_s] }.to_h # convert env keys and values to string
 
+      require "open3"
       Open3.popen3(env, cmd.to_s) do |stdin, stdout, stderr, wait_thr|
         yield stdin, stdout, wait_thr if block_given?
         stdin.close