Bump pyyaml from 3.12 to 5.4 in /docs

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.4)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -64,7 +64,7 @@ PySocks==1.6.7
 pytest==3.1.2
 python-dateutil==2.6.1
 pytz==2017.2
-PyYAML==3.12
+PyYAML==5.4
 pyzmq==16.0.3
 scikit-learn==0.19.1
 scipy==1.0.0