Calls to allowed_email_address_get return a string object which is not nice to evaluate the API response afterwards.
I used the following workaround to get a dictionary object for the response:
api_response = api_instance.allowed_email_address_get()
allowed = eval(api_response)
# now allowed.get('data') works
Would be best to get the dictionary object directly from allowed_email_address_get and not rely on being able to use eval for the returned string.