Upgrade graphviz dependency version

[elgalu]

mxnet/python/setup.py

Line 33 in b84609d

kwargs = {'install_requires': ['numpy>=1.17', 'requests>=2.20.0,<3', 'graphviz<0.9.0,>=0.8.1', 'contextvars;python_version<"3.7"'], 'zip_safe': False}

graphviz latest: 0.20.1

[github-actions]

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