Cask: add doctor check for xattr

CC: Homebrew/homebrew-cask#69660

Library/Homebrew/cask/cmd/doctor.rb

@@ -25,6 +25,7 @@ module Cask
 
       def run
         check_software_versions
+        check_xattr
         check_quarantine_support
         check_install_location
         check_staging_location
@@ -121,6 +122,29 @@ module Cask
         (locale_variables + environment_variables).sort.each(&method(:render_env_var))
       end
 
+      def check_xattr
+        ohai "xattr issues"
+        result = system_command "/usr/bin/xattr"
+
+        if result.status.success?
+          puts none_string
+        elsif result.stderr.match? "ImportError: No module named pkg_resources"
+          result = system_command "/usr/bin/python", "--version"
+
+          if result.stdout.match? "Python 2.7"
+            add_error "Your Python installation has a broken version of setuptools."
+            add_error "To fix, reinstall macOS or run 'sudo /usr/bin/python -m pip install -I setuptools'."
+          else
+            add_error "The system Python version is wrong."
+            add_error "To fix, run 'defaults write com.apple.versioner.python Version 2.7'."
+          end
+        elsif result.stderr.match? "pkg_resources.DistributionNotFound"
+          add_error "Your Python installation is unable to find xattr."
+        else
+          add_error "unknown xattr error: #{result.stderr.first}"
+        end
+      end
+
       def check_quarantine_support
         ohai "Gatekeeper support"