Bump pyyaml from 5.1 to 5.4 (#492)

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] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -2,7 +2,7 @@ sphinx==1.5.5
 recommonmark==0.4.0
 pypandoc==1.4
 pandocfilters==1.4.2
-PyYaml==5.1
+PyYaml==5.4
 mock==2.0.0
 javasphinx==0.9.15
 sphinxcontrib-apiblueprint==0.9.0