Issue #783: python3 compatible - remove relative imports.

I would like to avoid a massive patch, so I started publishing a series of commits to get rid of implicit relative imports.   Most changes are straightforward, but some may require discussion.  For example, changing foo.xyz to baz.bar.foo.xyz may result in a slight performance penalty.  I don't think that penalty is material, but there are ways to avoid it.  In any case, I would advocate "make it work first then make it fast" approach.