Return the time formatted according to ISO.
The full format looks like 'YYYY-MM-DD HH:MM:SS.mmmmmm'.
By default, the fractional part is omitted if self.microsecond == 0.
If self.tzinfo is not None, the UTC offset is also attached, giving
a full format of 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.
Optional argument sep specifies the separator between date and
time, default 'T'.
The optional argument timespec specifies the number of additional
terms of the time to include. Valid options are 'auto', 'hours',
'minutes', 'seconds', 'milliseconds' and 'microseconds'.