Bump pyyaml from 4.2b4 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 4.2b4 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

@@ -6,4 +6,4 @@ python-swiftclient==3.9.0
 python-keystoneclient==3.22.0
 keystoneauth1==3.14.0
 # error with 5.x (ConstructorError: could not determine a constructor for the tag '!record')
-PyYAML==4.2b4
+PyYAML==5.4