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

@@ -3,7 +3,7 @@ chardet==3.0.4
 idna==2.5
 pushbullet.py==0.11.0
 python-magic==0.4.13
-PyYAML==3.12
+PyYAML==5.1
 requests==2.18.1
 six==1.10.0
 urllib3==1.24.2