Bumps pyyaml from 5.3 to 5.4.

Sourced from pyyaml's changelog.

5.4 (2021-01-19)

• yaml/pyyaml#407 -- Build modernization, remove distutils, fix metadata, build wheels, CI to GHA
• yaml/pyyaml#472 -- Fix for CVE-2020-14343, moves arbitrary python tags to UnsafeLoader
• yaml/pyyaml#441 -- Fix memory leak in implicit resolver setup
• yaml/pyyaml#392 -- Fix py2 copy support for timezone objects
• yaml/pyyaml#378 -- Fix compatibility with Jython

5.3.1 (2020-03-18)

• yaml/pyyaml#386 -- Prevents arbitrary code execution during python/object/new constructor

• 58d0cb7 5.4 release
• a60f7a1 Fix compatibility with Jython
• ee98abd Run CI on PR base branch changes
• ddf2033 constructor.timezone: _copy & deepcopy
• fc914d5 Avoid repeatedly appending to yaml_implicit_resolvers
• a001f27 Fix for CVE-2020-14343
• fe15062 Add 3.9 to appveyor file for completeness sake
• 1e1c7fb Add a newline character to end of pyproject.toml
• 0b6b7d6 Start sentences and phrases for capital letters
• c976915 Shell code improvements
• Additional commits viewable in compare view

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