BlockTest.test_target calls not implemented block.difficulty()

[petercerno]

The following line should be removed:

https://github.com/jimmysong/programmingbitcoin/blob/2a6558263923214320bbdeb10826464fe24ef540/code-ch09/block.py#L153

The reason is that in Chapter9.ipynb in Exercise 9 one first implements the method bits_to_target(bits) and only then in Exercise 10 the method difficulty.