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

@@ -120,7 +120,7 @@ PyJWT==1.6.1
 python-dateutil==2.7.2
 python-editor==1.0.3
 pytz==2018.4
-PyYAML==3.12
+PyYAML==5.1
 requests==2.20.0
 requests-oauthlib==0.8.0
 SecretStorage==2.3.1