Update pytest to 3.8.0 (#11)

This PR updates [pytest](https://pypi.org/project/pytest) from **3.7.4** to **3.8.0**.



*The bot wasn't able to find a changelog for this release. [Got an idea?](https://github.com/pyupio/changelogs/issues/new)*

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

requirements.txt

@@ -3,6 +3,6 @@ jsonschema==2.6.0
 GitPython==2.1.11
 mistune==0.8.3
 flake8==3.5.0
-pytest==3.7.4
+pytest==3.8.0
 coveralls>=1.2.0
 requests==2.19.1