UndoItem merging of sequence types inconsistent

The merge method of `UndoItem` for the case of sequence objects returns `True` as long as only one item has changed from the original value:
```
class SimpleExample(HasTraits):
tuple_value = Tuple()

example = SimpleExample()

undo_item = UndoItem(
object=example,
name='tuple_value',
old_value=('foo', 'bar', 'baz'),
new_value=('foo', 'wombat', 'baz'),
)
next_undo_item = UndoItem(
object=example,
name='tuple_value',
old_value=('foo', 'wombat', 'baz'),
new_value=('foo', 'fizz', 'baz'),
)

result = undo_item.merge(next_undo_item)
```
In this case the `result` is `True`.

However, if any of the unchanging items is not a "simple type" (basically strings and numbers) then it always counts as a "changed item", so
```
undo_item = UndoItem(
object=example,
name='tuple_value',
old_value=(['foo'], 'bar', 'baz'),
new_value=(['foo'], 'wombat', 'baz'),
)
next_undo_item = UndoItem(
object=example,
name='tuple_value',
old_value=(['foo'], 'wombat', 'baz'),
new_value=(['foo'], 'fizz', 'baz'),
)

result = undo_item.merge(next_undo_item)
```
returns `False`.

Given that equality tests should work for almost all types, and the block catches all exceptions, it seems safe to simplify the code or otherwise make the behaviour more consistent.