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>

requirements.txt

@@ -25,7 +25,7 @@ pyflakes==2.1.1
 python-dateutil==2.8.0
 python-mimeparse==1.6.0
 pytz==2019.1
-PyYAML==5.1
+PyYAML==5.4
 requests==2.22.0
 six==1.12.0
 sqlparse==0.3.0