[FEA] treat warnings as errors and fix remaining (networkx, pandas, pytest)

[lmeyerov]

trafficstars

With https://github.com/graphistry/pygraphistry/pull/179/commits/0fc983e78a853971f06b29df00636490b2be35ec, we are down to only a few warnings -- pandas during df -> json, pytest not caching (RO docker vol), and networkx disabled. That means we can enable warnings-as-errors and look into the lingering ones.