Bump pyyaml from 5.1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -4,5 +4,5 @@ greenlet==0.4.13
 Mako==1.0.6
 MarkupSafe==1.1.1
 pathtools==0.1.2
-PyYAML==5.1
+PyYAML==5.4
 watchdog==0.8.3