Add require_env helper to python-order-utils README example

[TheAlonze01]

Add a small require_env helper to the README usage example so that
missing EXCHANGE_ADDRESS, CHAIN_ID or PRIVATE_KEY fail with a clear
error instead of a KeyError.

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Note

Update README usage to load EXCHANGE_ADDRESS, CHAIN_ID, and PRIVATE_KEY via a new require_env helper instead of hardcoded placeholders.

• Documentation (README):
  • Add require_env helper for fetching environment variables with clear errors.
  • Replace hardcoded exchange_address, chain_id, and signer setup with env-based values (EXCHANGE_ADDRESS, CHAIN_ID, PRIVATE_KEY), casting CHAIN_ID to int.

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