Pytest-xvfb

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- PyVirtualDisplay 1.3 and newer is now supported, support for older versions
was dropped.
- Support for Python 2.7, 3.3 and 3.4 is now dropped.
- Support for Python 3.6, 3.7 and 3.8 was added (no code changes required).
- Xvfb is now not started anymore in the xdist master process.

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- ``Item.get_closest_marker`` is now used, which restores compatibility with
pytest 4.1.0 and requires pytest 3.6.0 or newer.

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- The ``xvfb_args`` option is now a single line parsed with ``shlex.split``.
- The ``XvfbExitedError`` exception now includes stdout and stderr.

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- Use `PyVirtualDisplay`_ to start/stop Xvfb
- Show a warning on Linux if Xvfb is unavailable

.. _PyVirtualDisplay: https://pypi.python.org/pypi/PyVirtualDisplay

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- Add a new ``xvfb_xauth`` setting which creates an ``XAUTHORITY`` file.

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- The temporary directory searched for logfiles is now hardcoded to /tmp
as that's what X11 does as well.