Bump pyyaml from 3.11 to 5.1 in /docs

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.1)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1,4 +1,4 @@
 enum34==1.1.6
 Sphinx==1.6.2
 pika==0.10.0
-PyYAML==3.11
+PyYAML==5.1