Update pytest to 3.1.1

[pyup-bot]

There's a new version of pytest available.
You are currently using 3.1.0. I have updated it to 3.1.1

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

Changelog

3.1.1

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

Bug Fixes

• pytest warning capture no longer overrides existing warning filters. The
  previous behaviour would override all filters and caused regressions in test
  suites which configure warning filters to match their needs. Note that as a
  side-effect of this is that DeprecationWarning and
  PendingDeprecationWarning are no longer shown by default. (2430)

• Fix issue with non-ascii contents in doctest text files. (2434)

• Fix encoding errors for unicode warnings in Python 2. (2436)

• pytest.deprecated_call now captures PendingDeprecationWarning in
  context manager form. (2441)

Improved Documentation

• Addition of towncrier for changelog management. (2390)

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

Happy merging! 🤖