Bump pyyaml from 3.11 to 5.4 in /doc

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 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/compare/3.11...5.4)

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

doc/requirements.txt

@@ -12,7 +12,7 @@ MarkupSafe==0.23
 pathtools==0.1.2
 Pygments==2.1.1
 pytz==2015.7
-PyYAML==3.11
+PyYAML==5.4
 recommonmark==0.4.0
 singledispatch==3.4.0.3
 six==1.10.0