The NFT version that this contract uses.
ERC721 versions are:
1.0: CryptoKitties and early 721s, which lack approve-all and
have problems calling
transferFrom from the owner's account.
2.0: CryptoSaga and others that lack
transferFrom and have
3.0: The current OpenZeppelin standard:
locked: When the transfer function has been locked by the dev
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