Bump pyyaml from 3.12 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.1)

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

Author: dependabot[bot]

requirements.txt

@@ -46,7 +46,7 @@ python-dateutil==2.7.5
 pytz==2018.7
 pyverdict==0.1.3.2
 pyxdg==0.25
-PyYAML==3.12
+PyYAML==5.1
 requests==2.18.4
 scikit-learn==0.20.3
 scipy==1.2.1