Bump pyyaml from 5.1 to 5.4 in /doc

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]

doc/requirements.txt

@@ -14,7 +14,7 @@ pathtools==0.1.2
 port-for==0.3.1
 Pygments==2.1.3
 pytz==2016.1
-PyYAML==5.1
+PyYAML==5.4
 recommonmark==0.4.0
 singledispatch==3.4.0.3
 six==1.10.0