Bump pyyaml from 3.12 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.4)

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

requirements.txt

@@ -3,7 +3,7 @@ click==6.7
 packaging==16.8
 py==1.4.32
 pyparsing==2.1.10
-PyYAML==3.12
+PyYAML==5.4
 six==1.10.0
 backports.functools_lru_cache==1.0.1
 enum34==1.1.6