Minor Python 3.5+ improvements

[Stealthii]

This PR makes two small changes to the req_py3 branch:

Implement nested using contextmanager

A reimplementation of nested for Python 3 from 43cdd22, using the already imported contextmanager from contextlib.

This should silence linter warnings about the implementation method, but also be Python 3.4+ compatible and more future proof for changes in later Python versions.

Fix test_tasks for Python 3

This is a rework of https://github.com/ploxiln/fab-classic/commit/cadc78fbff372fd8e8050724da2c5c2a0be8382b that doesn't use six for Python 3.5+ support.