Fix decimal type in datetime.fromtimestamp

[RuVl]

python ./manage.py updatecurrencies oxr
raise TypeError: 'decimal.Decimal' object cannot be interpreted as an integer
on file: \venv\lib\site-packages\currencies\management\commands_openexchangerates.py", line 84, in get_ratetimestamp
return datetime.fromtimestamp(self.rates["timestamp"])

I fix this by replacing self.rates["timestamp"] to float(self.rates["timestamp"])