Mikhail Salnikov

Update NP_NAT defenition to fix TypeError bug

1

Solution for https://github.com/quantopian/trading_calendars/issues/224

TypeError bug with calendar_helpers NP_NAT

1

https://github.com/quantopian/trading_calendars/blob/19c4b677f13147928d34be5a3da50ba4161be45d/trading_calendars/calendar_helpers.py#L6 This definition leads to `TypeError: int() argument must be a string, a bytes-like object or a number, not 'NaTType'`