Bump pyyaml from 3.13 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.13 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.13...5.1)

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

Author: dependabot[bot]

requirements.txt

@@ -18,7 +18,7 @@ protobuf==3.6.1
 pyparsing==2.3.0
 python-dateutil==2.7.5
 pytz==2018.7
-PyYAML==3.13
+PyYAML==5.1
 scikit-learn==0.20.2
 scipy==1.2.0
 six==1.12.0