Retry on transient network errors in download_file (fixes Windows CI WinError 10054)

[Copilot]

Windows CI was intermittently failing with WinError 10054 (connection forcibly reset by remote host) when downloading datasets from raw.githubusercontent.com, causing 4 tests to fail despite no code defect.

Changes

• sacrebleu/utils.py — download_file: Wraps the urlopen call in a retry loop (3 attempts) with exponential backoff (2^attempt seconds) on OSError (which covers urllib.error.URLError and Windows socket errors). SSL errors still abort immediately.

for attempt in range(max_retries):
    try:
        with urllib.request.urlopen(source_path) as f, open(dest_path, 'wb') as out:
            out.write(f.read())
        break
    except ssl.SSLError:
        ...  # fatal, exit immediately
    except OSError as e:
        if attempt < max_retries - 1:
            sacrelogger.warning(f"Download attempt {attempt + 1} failed ({e}), retrying...")
            time.sleep(2 ** attempt)
        else:
            raise