docs: def sign_cart_for_env_maybe(self, cart, env=None):
          """
          Sign the items to upload, if the env requires a signature.
  
          `cart` - Cart to sign
          `envs` - The cart is signed if env has the property:
          requires_signature = True
  
          Will attempt to load the rpm_sign_plugin defined in
          ~/.config/juicer/config, which must be a plugin inheriting from
          juicer.common.RpmSignPlugin. If available, we'll call
          cart.sign_items() with a reference to the
          rpm_sign_plugin.sign_rpms method.
          """
          if self.connectors[env].requires_signature:
              cart.sync_remotes(force=True)
              juicer.utils.Log.log_notice("%s requires RPM signatures", env)
              juicer.utils.Log.log_notice("Checking for rpm_sign_plugin definition ...")
              module_name = self._defaults['rpm_sign_plugin']
              if self._defaults['rpm_sign_plugin']:
                  juicer.utils.Log.log_notice("Found rpm_sign_plugin definition: %s",
                                              self._defaults['rpm_sign_plugin'])
                  juicer.utils.Log.log_notice("Attempting to load ...")
  
                  try:
                      rpm_sign_plugin = __import__(module_name, fromlist=[module_name])
                      juicer.utils.Log.log_notice("Successfully loaded %s ...", module_name)
                      plugin_object = getattr(rpm_sign_plugin, module_name.split('.')[-1])
                      signer = plugin_object()
                      cart.sign_items(signer.sign_rpms)
                  except ImportError as e:
                      juicer.utils.Log.log_notice("there was a problem using %s ... error: %s",
                                                  module_name, e)
                      raise JuicerRpmSignPluginError("could not load rpm_sign_plugin: %s; additional information: %s" % \
                                                         (module_name,
                                                          e))
  
                  if not juicer.utils.rpms_signed_p([item.path for item in cart.items()]):
                      raise JuicerNotSignedError('RPMs have not been signed.')
  
              else:
                  raise JuicerConfigError("Did not find an rpm_sign_plugin in config file but the %s environment requires signed RPMs." % env)
              return True
          else:
              return None