Bump pyyaml from 4.2b1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 4.2b1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/commits/5.4)

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

requirements.txt

@@ -6,7 +6,7 @@ python-dateutil==2.5.3
 python-engineio==0.9.2
 python-mimeparse==1.5.2
 python-socketio==1.4.4
-PyYAML==4.2b1
+PyYAML==5.4
 requests==2.20.0
 Werkzeug==0.11.10
 httplib2==0.9.2