I got this working, however, the code I used is not fundamentally different from the snippet you posted. Just in case you'd like to diff my version with yours, attached below is a complete copy of a Python program that worked for me. I initially got a 403, just like you, which was due to inheriting your project id :). After updating that value to use my project ID, I got a correct bucket listing. Two things to check:
Make sure the project id you are using is correct and has the "Google Cloud Storage JSON API" enabled on the Google Developer Console "Services" tab (it's a different service from the other Google Cloud Storage API).
Make sure you are loading the service accounts private key exactly as it came from the developer's console. I would recommend reading it into memory from the file you downloaded, as I've done here, rather than trying to copy it into a string literal in your code.
#!/usr/bin/env python
import pprint
import oauth2client
from oauth2client.client import SignedJwtAssertionCredentials
import httplib2
from apiclient.discovery import build
f = open('key.p12', 'r')
key = f.read()
f.close()
credentials = SignedJwtAssertionCredentials(
'REDACTED',
key,
scope='https://www.googleapis.com/auth/devstorage.full_control')
http = httplib2.Http()
http = credentials.authorize(http)
service = build("storage", "v1beta1", http=http)
# Build the request
request = service.buckets().list(projectId="REDACTED")
# Diagnostic
pprint.pprint(request.headers)
pprint.pprint(request.to_json())
# Do it!
response = request.execute()
pprint.pprint(response)