Fix TraitListEvents for slice assignments with step != 1.

[mdickinson]

Slice assignments with a step != 1 currently emit a TraitListEvent whose attributes are wrapped in an extra list:

>>> from traits.api import *
>>> class A(HasTraits): foo = List(Int)
... 
>>> def foo_items_listener(new): print "List event: ", new.added, new.removed
... 
>>> a = A(foo=range(5)); a.on_trait_change(foo_items_listener, 'foo_items')
>>> a.foo[:] = range(4)
List event:  [0, 1, 2, 3] [0, 1, 2, 3, 4]
>>> a.foo[::2] = [23, 47]
List event:  [[23, 47]] [[0, 2]]

There are applications that make use of this behaviour, so we've deliberately avoided 'fixing' it so far. For Traits 5.0, it may be worth fixing it (with a corresponding noisy entry in the changelog, of course).

Code:

traits/traits/trait_handlers.py

Lines 2372 to 2384 in 46e9719

	if key.step == 1:
	# FIXME: Bug-for-bug compatibility with old __setslice__ code.
	# In this case, we return a TraitListEvent with an
	# index=key.start and the removed and added lists as they
	# are.
	index = key.start
	else:
	# Otherwise, we have an extended slice which was handled,
	# badly, by __setitem__ before. In this case, we return the
	# removed and added lists wrapped in another list.
	index = key
	values = [values]
	removed = [removed]

[mdickinson]

This was re-reported in #742, and fixed in #771