parse transaction fee correctly for okex3
parse transaction fee correctly for okex3

In `parse_transaction` of okex3,
[Line](https://github.com/ccxt/ccxt/blob/master/python/ccxt/okex3.py#L2099)
```
 fetchWithdrawals

     {
         amount: "4.72100000",
         withdrawal_id: "1729116",
         fee: "0.01000000eth",  ##  here the fee is endwith 'eth'
         currency: "ETH",
       ....
     }
```
the value of fee is end with  'currency'.
So the [code](https://github.com/ccxt/ccxt/blob/master/python/ccxt/okex3.py#L2140) cannot parse the real fee,
and the feeCost is None.
```
 feeCost = self.safe_float(transaction, 'fee')
```

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