Generate BlockHeader from state_root and parent_header

def generate_header_from_parent_header(
        compute_difficulty_fn: Callable[[BlockHeader, int], int],
        parent_header: BlockHeader,
        coinbase: Address,
        timestamp: Optional[int] = None,
        extra_data: bytes = b'') -> BlockHeader:
    """
    Generate BlockHeader from state_root and parent_header
    """
    if timestamp is None:
        timestamp = max(int(time.time()), parent_header.timestamp + 1)
    elif timestamp <= parent_header.timestamp:
        raise ValueError(
            "header.timestamp ({}) should be higher than"
            "parent_header.timestamp ({})".format(
                timestamp,
                parent_header.timestamp,
            )
        )
    header = BlockHeader(
        difficulty=compute_difficulty_fn(parent_header, timestamp),
        block_number=(parent_header.block_number + 1),
        gas_limit=compute_gas_limit(
            parent_header,
            gas_limit_floor=GENESIS_GAS_LIMIT,
        ),
        timestamp=timestamp,
        parent_hash=parent_header.hash,
        state_root=parent_header.state_root,
        coinbase=coinbase,
        extra_data=extra_data,
    )

    return header