Bump pyyaml from 5.3 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3...5.4)

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

Pipfile

@@ -16,5 +16,5 @@ jinja2 = "==2.10.1"
 icalendar = "==4.0.4"
 trans = "==2.1.0"
 mistune = "==0.8.4"
-pyyaml = "==5.3"
+pyyaml = "==5.4"
 flask-babel = "==1.0.0"