Description

A table that contains a mixin Column with Coordinates cannot be sorted.
I suspect that the underlying reason is the same as in #5745, but that issue is are least
listed as a caveat in the docs: http://docs.astropy.org/en/latest/table/mixin_columns.html#details-and-caveats

Unless there is work on SkyCoord under way, I suggest we add this issue to the caveats.
A related Feature request is: #6348

Expected behavior

Sort the table.

Actual behavior

TypeError: 'SkyCoord' object does not support item assignment

Steps to Reproduce

from astropy.coordinates import SkyCoord
from astropy.table import Table
import astropy.units as u

t = Table({'a':[2,1], 'b':SkyCoord(["1:12:43.2 +1:12:43", "1 12 43.2 +1 12 43"], unit=(u.deg, u.hourangle))})
t.sort('a')

System Details

Linux-3.10.0-1062.1.2.el7.x86_64-x86_64-with-centos-7.7.1908-Core
Python 3.7.3 (default, Mar 27 2019, 22:11:17)
[GCC 7.3.0]
Numpy 1.16.4
SciPy not installed
astropy 4.0.dev26779