fix future warnings

[gnzsnz]

@stefan-jansen , please review

this is to fix the FutureWarning on ledger.py:424

/home/gordon/.local/lib/python3.11/site-packages/zipline/finance/ledger.py:424: FutureWarning: Series.__setitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To set a value by position, use `ser.iloc[pos] = value`
  self.daily_returns_series[session_ix] = self.todays_returns

On branch future_warning Changes to be committed: modified: src/zipline/finance/ledger.py