Bug when trying to set the unit on a TimeSeries column

[astrofrog]

When trying to set the unit on a time series column, I am running into an error:

>>> from astropy import units as u
>>> from astropy.timeseries import TimeSeries
>>> ts1 = TimeSeries(time_start='2016-03-22T12:30:31', time_delta=3 * u.s, n_samples=5)
>>> ts1['flux'] = [1,2,3,4,5]
>>> ts1['flux'].unit = 's'
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-14-1ddfdfcdb27c> in <module>
      3 ts1 = TimeSeries(time_start='2016-03-22T12:30:31', time_delta=3 * u.s, n_samples=5)
      4 ts1['flux'] = [1,2,3,4,5]
----> 5 ts1['flux'].unit = 's'

~/Code/astropy/astropy/table/column.py in __setattr__(self, item, value)
    853             else:
    854                 if converted is not self:
--> 855                     self.parent_table.replace_column(self.name, converted)
    856 
    857     def _base_repr_(self, html=False):

~/Code/astropy/astropy/table/table.py in replace_column(self, name, col)
   1920             raise ValueError('cannot replace a table index column')
   1921 
-> 1922         t = self.__class__([col], names=[name])
   1923         cols = OrderedDict(self.columns)
   1924         cols[name] = t[name]

~/Code/astropy/astropy/timeseries/sampled.py in __init__(self, data, time, time_start, time_delta, n_samples, **kwargs)
     89 
     90         if time is None and time_start is None:
---> 91             raise TypeError("Either 'time' or 'time_start' should be specified")
     92         elif time is not None and time_start is not None:
     93             raise TypeError("Cannot specify both 'time' and 'time_start'")

TypeError: Either 'time' or 'time_start' should be specified

I haven't investigated this yet, but at first glance I'm surprised that simply setting the unit on a column causes a new TimeSeries to be instantiated.

[pllim]

Maintainer's note: timeseries was added in #8540 , so this fix only needs to be backported to 3.2 series and no earlier.