Courtney, you need to send the json as a kvp in the item_properties dictionary. You can read the json and pass it as a string (use json.dumps()). See below for an example.
import json
json_file = json.loads('your_json.json')
item_properties = {'title':'Web scene with photo realistic buildings',
'type':'Web Scene',
'snippet':'This scene highlights buildings of Montreal, Canada',
'tags':'ArcGIS Python API',
'text': json.dumps(json_file)}
web_scene_item = gis.content.add(item_properties)