When instanciating Steem() you can pass an array of wif keys:

steem = Steem(keys=[wif1, wif2, wif3]

of you install the keys into the library-internal wallet using pison

piston addkey

then you don't need to deal with keys in the script at all but need to provide the masterpassword for the wallet either in the terminal-prompt, or in the environmental variable UNLOCK