How to format text from custom actions to appear bold in Slack?

I have a bot running on Slack and some custom actions to cluster incoming data. I want to now bold the cluster names so that they are emphasized and easier to read, e.g.:

“Your Effects so far are clustered as follows:”

Landscape:

  • water resources will dry up
  • limitation of inhabitable land
  • draught

Political:

  • social injustice

The problem is that I cannot use markdown fuctions, since the data is processed in real-time, clusters are identified and then shown as output by the bot, so it cannot be a pre-defined response. Using color package or specifying color of the text only works in the console, but it not in Rasa X or Slack.

Here is the custom action:

class ActionMidSumEffects(Action):

    def name(self) -> Text:
        return "action_mid_sum_effects"

    def run(self, dispatcher: CollectingDispatcher,
            tracker: Tracker,
            domain: Dict[Text, Any]) -> List[Dict[Text, Any]]:

        print("Creating Mid Summary for Effects:")

        # Get Desired Output (only unique intents - merge same intents)
        midsum_effects_output = []

        for i in range(len(list_effects_intent)):           
            skip: bool = False
            for i2 in midsum_effects_output:                
                if i2['Intent'][0] == list_effects_intent[i]:    
                    print("Found Double Intents -> Setting skip=True -> Merging")
                    skip = True
                    i2['Texts'].append(list_effects_text[i])
            if not skip:                                       
                cur_intent = {'Intent': [list_effects_intent[i]], 'Texts': [list_effects_text[i]]}
                midsum_effects_output.append(cur_intent)
            print("Finished Creating Effects Output:")
            print(midsum_effects_output)

        out = "Your Effects so far are clustered as follows:\n"
        for i3 in midsum_effects_output:                                # Add Intents
            out = out + str(i3['Intent'][0]) + ":\n" 
            for i4 in i3['Texts']:                                      # Add Texts
                out = out + "        - " + (str(i4)) + "\n"
        print("Effects Output to Dispatcher:")
        print(out)

        dispatcher.utter_message(text=out)

        return []