chore(sdk/python): trim ethereum extras from web3 to eth-account

ethereum.py only imports eth_account. Pulling all of web3 (aiohttp,
websockets, requests, urllib3, ...) for one helper is wasteful — drop
~780 lines from the lockfile by depending on eth-account directly.

Users still get the same to_account / to_account_secure entrypoint with
no API change. The 'all' / 'eth' / 'ethereum' extras now resolve to
eth-account>=0.13.0.

Author: Leechael