Bump pyyaml from 3.11 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [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[bot] <support@github.com>

Author: dependabot[bot]

requirements_dev.txt

@@ -7,7 +7,7 @@ tox==2.3.1
 coverage==4.1
 Sphinx==1.4.8
 cryptography==1.7
-PyYAML==3.11
+PyYAML==5.1
 dill
 pyyaml
 scipy>=0.13.0