Actually, since the latest release of python-steem, the library also contains a wallet that can be managed through piston.
Just import your keys with

piston addkeys

and use

from steem import Steem

Since wif keys are stored encrypted, you will be asked to provide the password when voting. If you want to unlock the wallet from script, you can use the UNLOCK environmental variable:

UNLOCK="wallet-passphrase" python3 bot.py