Change codebase to use 4-space tabs, not hard tabs

Doctest doesn't get any indigestion from hard tabs, but as a developer, I sure get some from having to mix the two styles in my code. Not to mention that hard tabs make it difficult to edit on my phone over SSH, or to paste code here and there with predictable formatting. Once upon a time I was in love with hard tabs for their convenience on the keyboard, but these days it's too painful for other uses to maintain. Lop 'em all out, and replace them with 4 spaces each.