Approximate float comparison in test_math.py

TestMath.test_log fails on my system due to a float comparison issue:
```
    def test_log(self, space):
        w_res = space.execute("return [Math.log(4, 10), Math.log(28), Math.log(3, 4)]")
>       assert self.unwrap(space, w_res) == [math.log(4, 10), math.log(28), math.log(3, 4)]
E       assert [0.6020599913...4812503605781] == [0.60205999132...4812503605781]
E         At index 0 diff: 0.6020599913279623 != 0.6020599913279624
```

This branch switches to an approximate equality check. I don't know that this is the right way to solve the problem, but I couldn't find or think of anything better.