Bump pytest from 4.0.2 to 5.1.2

Bumps [pytest](https://github.com/pytest-dev/pytest) from 4.0.2 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/4.0.2...5.1.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-dev.txt

@@ -1,7 +1,7 @@
 -e .
 flake8==3.6.0
 async-timeout==3.0.1
-pytest==4.0.2
+pytest==5.1.2
 pytest-cov==2.7.1
 pytest-mock==1.10.4
 coverage==4.5.2