You need to follow the instructions here first: http://strava.github.io/api/v3/oauth/ Basically, the app you create has to authorize the user using it (in this case, you). I've written some example code below. I'm new to python so don't know how to automate log in, so you'll have to copy and paste the url to the browser and copy and paste the code in.
import requests
import json
#Replace #### with your client id (listed here: http://www.strava.com/settings/api)
#Replace &&&& with your redirect uri listed on the same page. I used localhost
#Go this url in your browser and log in. Click authorize
https://www.strava.com/oauth/authorize?client_id=###&response_type=code&redirect_uri=&&&&
#Copy and paste the code returned in the url
code='qwertyuio123456789'
#Replace @@@@ with the code on your api page
values={'client_id':'###', 'client_secret': '@@@@', 'code':code}
r = requests.post('https://www.strava.com/oauth/token', data=values)
json_string = r.text.replace("'", "\"")
values = json.loads(json_string)
#now you have an access token
r = requests.get('http://www.strava.com/api/v3/athletes/227615', params=values)
Have fun!