I have a web API that returns python dictionaries or lists as a response that I eval() in python scripts that use the API, for completness I wanted to set a proper content-type but not sure what would be best to use "text/x-python" or maybe "application/python", or something else? 
[edit] I'm also outputting JSON, I'm doing Python as an option mainly for internal use.[/edit]
I doubt there's an established MIME type.  Have you considered using JSON instead, it is almost the same as a Python dict, and has a better established culture of tools and techniques.
What mime-type should I return for a python string