Thanks for the review. I've made most of the changes you suggested, but there's
one thing I wanted to check about:

> - instead of "int(size)", use "size = size.__index__()" so as to forbid floats

The tests for readline() and readlines() expect a TypeError if size is None.
Calling size.__index__() in this case raises an AttributeError instead. Should I
change the tests to expect an AttributeError? Alternatively, something like this
would more closely match the behaviour of the old code:

    try:
        size = size.__index__()
    except AttributeError:
        raise TypeError("Integer argument expected")