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Localhost is translated to 127.0.0.1 but if you got a 503 error, it means you have another HTTP server replying on port 8000.
You could have another instance running. (Apache, other Django instance, etc) Check twice.
Verify with netstat if needed.
And then try to run:
python manage.py runserver 0.0.0.0:8000
To bind the dev server on every addresses.
If it doesn’t work, try to launch the devserver again on a different port.
Source:stackexchange.com