Install py.test>=2.8,<3

The 3.x series is currently broken. Until py.test works again, 3.x
should not be used by anything.

Make Extension accept positional arguments again (fixes #752).

Fix issue #459

Put import on top and updated CHANGES.rst

- have tox pass env variables used by setuptools on windows.

As a side effect, py_limited_api may now only be passed as keyword
argument. I think it is early enough for this feature to change this
and enforce the flag to be passed as a keyword...

Fix quantity of blank lines after code object, class of function definition.

use abi3 extension if Extension().is_abi3