Bump pyyaml from 3.11 to 5.1

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] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -7,6 +7,6 @@ mailchimp==2.0.7
 wsgiref==0.1.2
 cronfed==0.2.1
 ashes==0.7.6
-PyYAML==3.11
+PyYAML==5.1
 python-dateutil==2.4.2
 Babel==2.0