Bitsv

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- Update bchsvexplorer.com to bsvbook.guarda.co c/o `xloem <https://github.com/xloem>`_ and `kcentrifugal <https://github.com/kcentrifugal>`_
- Extend test coverage to py38 and py39

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- Make 'prepare_transaction' a classmethod as it should not require a private key (breaking change as function signature now requires the 'network' as a positional argument)

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- NetworkAPI.broadcast_tx() now returns the txid instead of null.

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- Fixed an off-by-one bug for generating pushdata op_codes for data-carrier 'op_return' txs - c/o `gitzhou <https://github.com/gitzhou>`_.

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- Added WhatsOnChain API for mainnet and testnet (MatterCloud is now a paid-only service)
- Unspent data type: removed 'script' (scriptpubkey) attribute (unavailable from main WhatsOnChain and Satoshi.io APIs and is not even used for anything in the codebase at present)
- Transaction, TxOutput, TxInput data types: removed unnecessary 'cruft' like 'amount_in', 'amount_out' and 'fee' (also to facilitate use of WhatsOnChain etc. and reduce maintenance overheads).
- MatterCloud now activated via **'MATTERCLOUD_API_KEY' environment variable** which will make it the highest priority API in the list.
- Renamed BitIndex3 to MatterCloud

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- Update main api url from https://api.bitindex.network to https://api.mattercloud.net