Being on Python, you have the great advantage of having access to NLTK :-) http://www.nltk.org/ - it's pretty intense and offers a lot, but also includes some example bots: http://www.nltk.org/api/nltk.chat.html

My personal advice, though, is while you can definitely try to support a more 'natural' type of language, you do want to define fixed formats for commands in your documentation/help so as not to confuse users or encourage them to keep trying all sorts of variations to figure out what magic combo makes your bot work.

You might find AIML useful. I don't know much about the state of AIML interpreters in Python, but I'm guessing there is at least one good one.

www.converse.ai has an API you can use. Works cross platform - gives you Slack, SMS, chat etc out the box.