Return the time formatted according to ISO.
The full format is 'HH:MM:SS.mmmmmm+zz:zz'. By default, the
fractional part is omitted if self.microsecond == 0.
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'.