Inconsistent code snippet imports

[matteosantama]

pg. 119 has the following code snippet

from typing import Iterator
X = TypeVar('X')

def iterate(step: Callable[[X], X], start: X) -> Iterator[X]:
    ...

It seems odd to explicitly import Iterator from typing, but not Callable or TypeVar.

[matteosantama]

Looks like this will be closed by #61

[coverdrive]

Thanks. We need to do a pass through the book to handle these inconsistencies in what imports are shown and what aren't.