gradient-cli PyYAML 5.4.1 is broken, use PyYAML 5.3.1 instead

PyYAML version 5.4.1 is causing 'pip install gradient' to fail.

See: https://github.com/yaml/pyyaml/issues/724

Solution is to use 5.3.1

ie., change line 70 in setup.py from PyYAML==5.* to PyYAML==5.3.*

Collecting PyYAML==5.* (from gradient)
  Using cached PyYAML-5.4.1.tar.gz (175 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [62 lines of output]
      running egg_info
      writing lib3/PyYAML.egg-info/PKG-INFO
      writing dependency_links to lib3/PyYAML.egg-info/dependency_links.txt
      writing top-level names to lib3/PyYAML.egg-info/top_level.txt
      /private/var/folders/fd/b8flfqkj3_bf4hp3dnr1l2hh0000gn/T/pip-build-env-usxd2f4p/overlay/lib/python3.8/site-packages/setuptools/config/setupcfg.py:293: _DeprecatedConfig: Deprecated config in `setup.cfg`
      !!
      
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