Update PyYAML to 5.4.1 to fix installation error on Python3.9

[kga]

furoshiki2 just using safe_load, so it won't be a problem to update PyYAML. It worked in my local environment with python@3.9: stable 3.9.1 (bottled)

• https://github.com/yaml/pyyaml/releases?after=5.1b1
  • The next version after 3.13 is 5.1 because 4.1 is retracted.
• Incompatible changes in version 5.1 yaml/pyyaml#265
• .load() and FullLoader still vulnerable to fairly trivial RCE yaml/pyyaml#420 (comment)
• url and sha256
  • https://pypi.org/project/PyYAML/5.4.1/#files