Very simple:

import json
data = json.loads('{"one" : "1", "two" : "2", "three" : "3"}')
print(data['two'])  # or `print data['two']` in Python 2

You are ignoring the return value of json.loads() here:

json.loads(r.data.decode('utf-8'))

You then try to decode the same raw again and try to use that as the decoded Python result. Call json.loads() once, and use the resulting Python dictionary:

result = json.loads(r.data.decode('utf-8'))
start = result['data'][0]['start_time_delta']
end = result['data'][0]['end_time_delta']

Because top-level dictionary 'data' key points to a list of results, I used 0 to get to the first of those and extract the data you want.

If you need to extract those data points for every dictionary in that list, you'd have to use a loop:

for entry in result['data']:
    start = entry['start_time_delta']
    end = entry['end_time_delta']
    # do something with these two values, before iterating to the next