Bump pyyaml from 3.13 to 5.1 in /tn/generate_tn_pdf

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.13 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.13...5.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tn/generate_tn_pdf/requirements.txt

@@ -7,7 +7,7 @@ Markdown==3.0.1
 markdown2==2.3.6
 pyparsing==2.1.10
 python-dateutil==2.3
-PyYAML==3.13
+PyYAML==5.1
 singledispatch==3.4.0.3
 six==1.11.0
 tornado==5.1.1