TST: Key trading calendar fixture with Asset types
TST: Key trading calendar fixture with Asset types

Instead of using strings of 'equities' and 'futures', use the Asset
subclasses to key the trading calendar fixtures.

alphaBenj/zipline,Scapogo/zipline,quantopian/zipline,magne-max/zipline-ja,bartosh/zipline,alphaBenj/zipline,humdings/zipline,bartosh/zipline,enigmampc/catalyst,magne-max/zipline-ja,florentchandelier/zipline,florentchandelier/zipline,enigmampc/catalyst,humdings/zipline,Scapogo/zipline,quantopian/zipline