Update pytest to 3.1.2

[pyup-bot]

There's a new version of pytest available.
You are currently using 3.1.1. I have updated it to 3.1.2

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

Changelog

3.1.2

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

Bug Fixes

• Required options added via pytest_addoption will no longer prevent using
  --help without passing them. (1999)

• Respect python_files in assertion rewriting. (2121)

• Fix recursion error detection when frames in the traceback contain objects
  that can't be compared (like numpy arrays). (2459)

• UnicodeWarning is issued from the internal pytest warnings plugin only
  when the message contains non-ascii unicode (Python 2 only). (2463)

• Added a workaround for Python 3.6 WindowsConsoleIO breaking due to Pytests's
  FDCapture. Other code using console handles might still be affected by the
  very same issue and might require further workarounds/fixes, i.e. colorama.
  (2467)

Improved Documentation

• Fix internal API links to pluggy objects. (2331)

• Make it clear that pytest.xfail stops test execution at the calling point
  and improve overall flow of the skipping docs. (810)

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

Happy merging! 🤖