Reading from an `Event` trait inside a `Tuple` trait should be an error

Reading the value of an `Event()` trait should be an error, even when it's contained in another trait type. But we have the following behaviour in current Traits:
```python
>>> from traits.api import HasTraits, Event, Tuple
>>>
>>> class A(HasTraits):
... foo = Tuple(Event(), Event())
...
>>> a = A()
>>> a.foo
(, )
```