Chapter 9 exercise 9 test is testing too much

[bolatovumar]

Test for exercise 9 in chapter 9 tests for block target and difficulty. It should test for block target only. Exercise for difficulty is exercise 10.

This line should be removed: https://github.com/jimmysong/programmingbitcoin/blob/master/code-ch09/block.py#L153

[lucidprojects]

Thank you 🙏