Here is one way to do it. Extract the hour from current time, and initialize a new datetime.time object with hour as input. Then you convert both of the timestamps to datetime and then do the subtraction.
Code below does that
Time = datetime.datetime.now().time()
hour = str(Time).split(":")[0]
currentHourTime = datetime.datetime(2019,10,31,int(hour),0,0,0).time()
dateTimeCurr = datetime.datetime.combine(datetime.date.today(), Time)
dateTimeCurrHour = datetime.datetime.combine(datetime.date.today(), currentHourTime)
dateTimeDifference = dateTimeCurr - dateTimeCurrHour
dateTimeDifferenceInSeconds = dateTimeDifference.total_seconds()
print(dateTimeDifferenceInSeconds)