Bump pyyaml from 5.3.1 to 5.4 (#89)

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/5.3.1...5.4)

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

requirements-travis.txt

@@ -53,7 +53,7 @@ PyNaCl==1.3.0
 pytest==4.3.1
 python-dateutil==2.8.1
 python-gilt==1.2.3
-PyYAML==5.3.1
+PyYAML==5.4
 requests==2.23.0
 ruamel.yaml==0.16.10
 ruamel.yaml.clib==0.2.0