Loosen Graphviz version requirements

[huubvh95]

Description

MXnet depends on a very old version of Graphviz and it sets this version as the upper bound. Is it possible to loosen these requirements?

References

• current requirement: graphviz<0.9.0,>=0.8.1
• current version: graphviz==0.19.1

[github-actions]

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[melanopsis]

Currently, graphviz is at v0.20.1. The version that mxnet brings (v0.8.4) was released on Jul 1, 2018.