Pytest-xdist

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- fix 124: xdist would mark test as complete after 'call' step. As a result,
xdist could identify the wrong test as failing when test crashes at teardown.
To address this issue, xdist now marks test as complete at teardown.

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- ``pytest-xdist`` now requires pytest 2.7 or later.

- Add ``worker_id`` attribute in the TestReport

- new hook: ``pytest_xdist_make_scheduler(config, log)``, can return custom tests items
distribution logic implementation. You can take a look at built-in ``LoadScheduling``
and ``EachScheduling`` implementations. Note that required scheduler class public
API may change in next ``pytest-xdist`` versions.

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- new ``worker_id`` fixture, returns the id of the worker in a test or fixture.
Thanks Jared Hellman for the PR.

- display progress during collection only when in a terminal, similar to pytest 1397 issue.
Thanks Bruno Oliveira for the PR.

- fix internal error message when ``--maxfail`` is used (62, 65).
Thanks Collin RM Stocks and Bryan A. Jones for reports and Bruno Oliveira for the PR.

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- new hook: ``pytest_xdist_node_collection_finished(node, ids)``, called when
a worker has finished collection. Thanks Omer Katz for the request and
Bruno Oliveira for the PR.

- fix README display on pypi

- fix 22: xdist now works if the internal tmpdir plugin is disabled.
Thanks Bruno Oliveira for the PR.

- fix 32: xdist now works if looponfail or boxed are disabled.
Thanks Bruno Oliveira for the PR.

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- fix a regression -n 0 now disables xdist again

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- extended the tox matrix with the supported py.test versions

- split up the plugin into 3 plugin's
to prepare the departure of boxed and looponfail.

looponfail will be a part of core
and forked boxed will be replaced
with a more reliable primitive based on xdist

- conforming with new pytest-2.8 behavior of returning non-zero when all
tests were skipped or deselected.

- new "--max-slave-restart" option that can be used to control maximum
number of times pytest-xdist can restart slaves due to crashes. Thanks to
Anatoly Bubenkov for the report and Bruno Oliveira for the PR.

- release as wheel

- "-n" option now can be set to "auto" for automatic detection of number
of cpus in the host system. Thanks Suloev Dmitry for the PR.