Update dependency PyYAML to v5.4

[mend-for-github-com]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
PyYAML (source)	==5.1 -> ==5.4

By merging this PR, the issue #7 will be automatically resolved and closed:

Severity	CVSS Score	CVE
Critical	9.8	CVE-2019-20477
Critical	9.8	CVE-2020-14343
Critical	9.8	CVE-2020-1747

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Release Notes

yaml/pyyaml (PyYAML)

v5.4

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v5.3.1

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v5.3

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v5.2

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• Repair incompatibilities introduced with 5.1. The default Loader was changed,
  but several methods like add_constructor still used the old defahttps://github.com/yaml/pyyaml/pull/279pull/279 -- A more flexible fix for custom tag consthttps://github.com/yaml/pyyaml/pull/287aml/pull/287 -- Change default loader for yaml.add_https://github.com/yaml/pyyaml/pull/305/pyyaml/pull/305 -- Change default loader for add_implicit_resolver, add_path_resolver
• Make FullLoader safer by removing python/object/apply from the default FullLoader
  https://github.com/yaml/pyyaml/pull/347/347 -- Move constructor for object/apply to UnsafeConstructor
• Fix bug introduced in 5.1 where quoting went wrong on systems with sys.maxunicode <= 0xffff
  https://github.com/yaml/pyyaml/pull/276/276 -- Fix logic for quoting special characters
• Other PRs:
  https://github.com/yaml/pyyaml/pull/280/280 -- Update CHANGES for 5.1

v5.1.2

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• Re-release of 5.1 with regenerated Cython sources to build properly for Python 3.8b2+

v5.1.1

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• Re-release of 5.1 with regenerated Cython sources to build properly for Python 3.8b1

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