Altered-states

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* Fixed issue in setup.py script
* Added missing changelog

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* Altered States now runs on Python 3 (tested on 2.7, 3.5 and 3.6)
* Dropped support for Python 2.6
* Experimental support for Kenneth Reitz Pipenv tool
* Corrected invalid use of `os.modules` in examples

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* Better handling of objects that override `__getitem__` (thanks to
merwok).
* Drop support for Python 2.5 (no sane way to solve issue 4 there).

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* Added a new API entry point: `alter()`, that can be used to perform
a two-step reversible alteration.

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* Updated test suites to use `pytest.fixture` notation for fixtures
(now requires `py.test` > 2.3)
* Fixes a bug causing `os.environ` not to be patchable.

Fixing bug 2 means switching the `dict` -like object check from
`isinstance(x, dict)` to `hasattr(x, '__getitem__')`. This change is
thought to not break backwards compatibility but if you encounter
unexpected behaviour in `dict` / `object` detection this might be
it. I'd be very interested to know about that if you do.

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* Alias `Expando` as `E` for optional terseness.