Support item assignment

Since OrderedSet supports indexing, it’s natural to assume that assignments and del would work as well, but they don’t:

>>> s = OrderedSet(['foo'])
>>> s[0]
'foo'
>>> s[0] = 'bar'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'OrderedSet' object does not support item assignment
>>> del s[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'OrderedSet' object doesn't support item deletion

There’s an easy workaround for del (s.remove(s[0])), but not for item assignments. As far as I can tell, there is no way to reorder items at all, so it’s impossible to replace an element with another one (while preserving its index).

[rspeer]

I think the only reasonable thing to do with __setitem__ given an existing item is to raise an error. That makes __setitem__ actually the easier case, so I can support it too.

[WillDaSilva]

I agree with @Aran-Fey’s breakdown. __delitem__ seems straightforward. As for __setitem__, 2.ii and 2.iii doing anything other than raising an exception (ValueError) would be surprising, but we can probably support 2.i without much difficulty or confusion. Since membership checking is O(1), checking if an exception must be raised for __setitem__ would be efficient.

It might be simpler to not support __setitem__, but if nothing else we can probably move forwards with __delitem__.