Bump pyyaml from 5.3.1 to 5.4 in /tests (#434)

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](yaml/pyyaml@5.3.1...5.4)

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

tests/requirements.txt

@@ -5,7 +5,7 @@ lxml==4.6.3
 requests==2.22.0
 pandas==1.0.1
 beautifulsoup4==4.9.0
-pyyaml==5.3.1
+pyyaml==5.4
 pytest-cov==2.10.0
 coveralls==2.1.1
 sqlalchemy==1.4.1