Update dependency pyyaml to v5

[dev-mend-for-github-com]

This PR contains the following updates:

Package	Update	Change
pyyaml (source)	major	==3.13 → ==5.4

By merging this PR, the issue #6 will be automatically resolved and closed:

Severity	CVSS Score	Vulnerability
Critical	9.8	CVE-2017-18342
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 default
  #​279 -- A more flexible fix for custom tag constructors
  #​287 -- Change default loader for yaml.add_constructor
  #​305 -- Change default loader for add_implicit_resolver, add_path_resolver
• Make FullLoader safer by removing python/object/apply from the default FullLoader
  #​347 -- Move constructor for object/apply to UnsafeConstructor
• Fix bug introduced in 5.1 where quoting went wrong on systems with sys.maxunicode <= 0xffff
  #​276 -- Fix logic for quoting special characters
• Other PRs:
  #​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

v5.1

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

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