Get current balance from Fido.

def _get_balance(self, account_number):
        """Get current balance from Fido."""
        # Prepare data
        data = {"ctn": self.username,
                "language": "en-US",
                "accountNumber": account_number}
        # Http request
        try:
            raw_res = yield from self._session.post(BALANCE_URL,
                                                    data=data,
                                                    headers=self._headers,
                                                    timeout=self._timeout)
        except OSError:
            raise PyFidoError("Can not get balance")
        # Get balance
        try:
            json_content = yield from raw_res.json()
            balance_str = json_content\
                            .get("getAccountInfo", {})\
                            .get("balance")
        except (OSError, ValueError):
            raise PyFidoError("Can not get balance as json")
        if balance_str is None:
            raise PyFidoError("Can not get balance")
        # Casting to float
        try:
            balance = float(balance_str)
        except ValueError:
            raise PyFidoError("Can not get balance as float")

        return balance