Update pytest to 3.6.4

[pyup-bot]

This PR updates pytest from 3.6.3 to 3.6.4.

Changelog

3.6.4

=========================

Bug Fixes
---------

- Invoke pytest using ``-mpytest`` so ``sys.path`` does not get polluted by packages installed in ``site-packages``. (`742 <https://github.com/pytest-dev/pytest/issues/742>`_)


Improved Documentation
----------------------

- Use ``smtp_connection`` instead of ``smtp`` in fixtures documentation to avoid possible confusion. (`3592 <https://github.com/pytest-dev/pytest/issues/3592>`_)


Trivial/Internal Changes
------------------------

- Remove obsolete ``__future__`` imports. (`2319 <https://github.com/pytest-dev/pytest/issues/2319>`_)

- Add CITATION to provide information on how to formally cite pytest. (`3402 <https://github.com/pytest-dev/pytest/issues/3402>`_)

- Replace broken type annotations with type comments. (`3635 <https://github.com/pytest-dev/pytest/issues/3635>`_)

- Pin ``pluggy`` to ``<0.8``. (`3727 <https://github.com/pytest-dev/pytest/issues/3727>`_)

Links

• PyPI: https://pypi.org/project/pytest
• Changelog: https://pyup.io/changelogs/pytest/
• Repo: https://github.com/pytest-dev/pytest/issues
• Homepage: http://pytest.org