Bump pyyaml from 5.1 to 5.4 in /tools/pdtools

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]

tools/pdtools/requirements.txt

@@ -11,7 +11,7 @@ GitPython==2.1.5
 idna==2.8
 jsonschema==2.6.0
 python-dateutil==2.7.5
-PyYAML==5.1
+PyYAML==5.4
 requests==2.21.0
 six==1.10.0
 smmap2==2.0.3