Remove use of ``from future import standard_library`` for now (issue #826)

This patch removes the use of the ``from future import standard_library`` feature. This line was actually redundant in most of the modules using it anyway, particularly because the feature doesn't yet support remapping of ``urllib``, so these cases were handled manually using ``try`` / ``except`` anyway.

I have posted to python-list about a safer mechanism for import hooks for the ``standard_library`` feature in ``future``, and I'll cross-post it to the Import-SIG list.

Thanks, @AlexHill, for the heads-up!