Update pytest to 3.9.2 (#14)

This PR updates [pytest](https://pypi.org/project/pytest) from **3.9.1** to **3.9.2**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.9.2
   ```
   =========================

Bug Fixes
---------

- `2909 &lt;https://github.com/pytest-dev/pytest/issues/2909&gt;`_: Improve error message when a recursive dependency between fixtures is detected.


- `3340 &lt;https://github.com/pytest-dev/pytest/issues/3340&gt;`_: Fix logging messages not shown in hooks ``pytest_sessionstart()`` and ``pytest_sessionfinish()``.


- `3533 &lt;https://github.com/pytest-dev/pytest/issues/3533&gt;`_: Fix unescaped XML raw objects in JUnit report for skipped tests


- `3691 &lt;https://github.com/pytest-dev/pytest/issues/3691&gt;`_: Python 2: safely format warning message about passing unicode strings to ``warnings.warn``, which may cause
  surprising ``MemoryError`` exception when monkey patching ``warnings.warn`` itself.


- `4026 &lt;https://github.com/pytest-dev/pytest/issues/4026&gt;`_: Improve error message when it is not possible to determine a function&#39;s signature.


- `4177 &lt;https://github.com/pytest-dev/pytest/issues/4177&gt;`_: Pin ``setuptools&gt;=40.0`` to support ``py_modules`` in ``setup.cfg``


- `4179 &lt;https://github.com/pytest-dev/pytest/issues/4179&gt;`_: Restore the tmpdir behaviour of symlinking the current test run.


- `4192 &lt;https://github.com/pytest-dev/pytest/issues/4192&gt;`_: Fix filename reported by ``warnings.warn`` when using ``recwarn`` under python2.
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/pytest
  - Changelog: https://pyup.io/changelogs/pytest/
  - Homepage: https://docs.pytest.org/en/latest/
</details>

Author: pyup-bot

Diff for: requirements_dev.txt

@@ -8,6 +8,6 @@ coverage==4.5.1
 Sphinx==1.8.1
 twine==1.12.1
 
-pytest==3.9.1
+pytest==3.9.2
 pytest-runner==4.2
 pytest-watch==4.2.0