Drug with as many targets as possible through a system biology graph?
I don't know any python (or R package) to do this), but this is how I would do it based on these matrices:


Do a multidimensional scaling (MDS) plot of the target_similarity matrix.  Explore the visualization: 
Find which drug targets the drug closest to the 0,0 point in the 2D space


However this doesn't say anything about the affinity between the drug and the target.
If you wanted to add this then you could weight how much close is the distance between the drug and the target vs how close are they to the center of the MDS (a simple division could work if the distance is scaled between 0 and 1 and averaged).

Note that depending on the original information used to calculate the similarities and the affinities it could be done in other ways