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)

Author: dependabot[bot]

requirements.txt

@@ -23,7 +23,7 @@ python-dateutil==2.8.0
 python-jose==3.0.1
 python-u2flib-server==5.0.0
 pytz==2018.9
-PyYAML==3.13
+PyYAML==5.1
 rsa==3.4.2
 s3transfer==0.2.0
 simplejson==3.16.0