Update pytest to 3.1.0

[pyup-bot]

There's a new version of pytest available.
You are currently using 3.0.7. I have updated it to 3.1.0

These links might come in handy: PyPI | Changelog | Repo | Homepage

Changelog

3.0.8

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

• Change capture.py's DontReadFromInput class to throw io.UnsupportedOperation errors rather
  than ValueErrors in the fileno method (2276).
  Thanks metasyn for the PR.

• Fix exception formatting while importing modules when the exception message
  contains non-ascii characters (2336).
  Thanks fabioz for the report and nicoddemus_ for the PR.

• Added documentation related to issue (1937)
  Thanks skylarjhdownes for the PR.

• Allow collecting files with any file extension as Python modules (2369).
  Thanks Kodiologist for the PR.

• Show the correct error message when collect "parametrize" func with wrong args (2383).
  Thanks The-Compiler for the report and robin0371_ for the PR.

.. _skylarjhdownes: https://github.com/skylarjhdownes
.. _fabioz: https://github.com/fabioz
.. _metasyn: https://github.com/metasyn
.. _Kodiologist: https://github.com/Kodiologist
.. _robin0371: https://github.com/robin0371

.. _1937: pytest-dev/pytest#1937
.. _2276: pytest-dev/pytest#2276
.. _2336: pytest-dev/pytest#2336
.. _2369: pytest-dev/pytest#2369
.. _2383: pytest-dev/pytest#2383

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

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This change is 

[pyup-bot]

Closing this in favor of #142