Bump `python-bitcoinlib` Version to Match `counterparty-lib` Dependency

[herrmoltke]

The version of counterblock's python-bitcoin-lib dependency should match counterparty-lib's.

counterblock requires v0.7.0 of python-bitcoinlib, while counterparty-lib requires v0.8.1. v7.0 doesnt support segwit and so if you have counterparty-lib and counterblock installed on the same machine, you will have to downgrade python-bitcoinlib in order to satisfy counterblock's dependency on it, which will halt counterparty-server, like in this issue: https://github.com/CounterpartyXCP/counterparty-lib/issues/1028

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