added reference to JanusGraph-Python package

[pm-osc]

I have noticed that in the latest JanusGraph documentation the JanusGraph-Python package is not mentioned on the https://docs.janusgraph.org/basics/connecting/python/ page. With this PR, I have added the missing reference following the format used for JanusGraph.Net.

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[porunov]

LGTM. Thank you @pm-osc !

[li-boxuan]

Thank you!