Looks like you need to truncate your microseconds to 6 decimal places (documentation seems to support this: https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior)
The following worked fine:
import datetime
d = datetime.datetime.strptime('2018-11-20T09:12:01.751171Z', '%Y-%m-%dT%H:%M:%S.%fZ')
If you want to correctly round your microseconds, try this:
import datetime
time_string = '2018-11-20T09:12:01.7511709Z'
date_time, microseconds = time_string.split('.')
microseconds = microseconds[:-1]
rounding = len(microseconds) - 6
divisor = 10 ** rounding
new_micros = int(round(int(microseconds) / divisor, 0))
time_string = date_time + '.' + str(new_micros) + 'Z'
d = datetime.datetime.strptime(time_string, '%Y-%m-%dT%H:%M:%S.%fZ')