Python Exchange never updates markets_by_slug

[ndmeiri]

The Python Exchange class defines an attribute called markets_by_slug but never adds items to it.

Presumably, markets_by_slug should be updated in Exchange.load_markets, which is where Exchange.markets is updated to index the markets by market ID.

[ndmeiri]

I noticed that the TypeScript Exchange does not have an analogue. So perhaps this functionality was meant to be removed from the Python side? I opened #103 but let me know if markets_by_slug should be removed.