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
```python
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