__imul__ with a List of List is inconsistent with Python __imul__

Consider this, where the list is an instance of `TraitListObject`:
```
from traits.api import HasTraits, List
class Foo(HasTraits):
a = List(List)

>>> foo = Foo(a=[[1, 2, 3], [3, 4, 5]])
>>> foo.a *= 2
[[1, 2, 3], [3, 4, 5], [1, 2, 3], [3, 4, 5]]
>>> foo.a
>>> foo.a[-1].append(6)
>>> foo.a
[[1, 2, 3], [3, 4, 5], [1, 2, 3], [3, 4, 5, 6]]
```
`TraitListObject.__imul__` creates copies for the "new" items.

For Python's list:
```
>>> a = [[1, 2, 3], [3, 4, 5]]
>>> a *= 2
[[1, 2, 3], [3, 4, 5], [1, 2, 3], [3, 4, 5]]
>>> a
>>> a[-1].append(6)
>>> a
[[1, 2, 3], [3, 4, 5, 6], [1, 2, 3], [3, 4, 5, 6]]
```
Note the difference in the second item in the final list. Python list `__imul__` simply repeats the items in the list.