Remove discussion of `is` in "Write Code for People"

It's rarely-used (I write a lot of python and basically never use it) and really special-case-y. It's mostly useful for `if foo is None` but we don't ever cover `None`, so there's that. And `is` doesn't come up in any of the rest of the curriculum.

Anyhow, we never cover whitespace in the current version, (though we claim otherwise!) so we'll need space from somewhere.