First you need to parse the time string. You can do that by hand, or use datetime.strptime():
from datetime import datetime
s = u'08:00:00'
t = datetime.strptime(s, '%H:%M:%S').time()
strptime() doesn't care that you give it a unicode string. That will give you a datetime.time object:
>>> t
datetime.time(8, 0)
Then use datetime.combine() to combine the time with today's date:
from datetime import date
dt = datetime.combine(date.today(), t)
This will give you the required datetime.datetime object:
>>> dt
datetime.datetime(2017, 2, 24, 8, 0)