Bump pyyaml from 5.1 to 5.4

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]

requirements.txt

@@ -25,7 +25,7 @@ Pygments==2.2.0
 pylint==1.8.4
 pytest-runner==4.2
 python-dateutil==2.6.1
-PyYAML==5.1
+PyYAML==5.4
 pytz==2020.1
 requests==2.20.0
 singledispatch==3.4.0.3