[Security] Bump pyyaml from 3.11 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1. **This update includes security fixes.**
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.1)

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

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -7,4 +7,4 @@ tox==2.3.1
 coverage==4.1
 Sphinx>=1.4,<1.5
 cryptography==2.3.1
-PyYAML==3.11
+PyYAML==5.1