Bump pytest from 5.0.1 to 5.1.2 (#917)

Bump [pytest](https://github.com/pytest-dev/pytest) from 5.0.1 to 5.1.2.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/5.0.1...5.1.2)

test-requirements.txt

@@ -1,4 +1,4 @@
-pytest==5.0.1
+pytest==5.1.2
 pytest-cov==2.7.1
 coverage==4.5.4
 pre-commit==1.18.3