Make TraitListObject notify even if list values stay the same

[ievacerny]

Closes #972

This PR changes the behaviour of TraitListObject. From now on it will notify even if the underlying list doesn't change (it didn't before). An exception - notification is still suppressed if there is no structural change.

For example, given a class:

>>> class Foo(HasTraits):
...     bar = List([1, 2, 3])
...     @on_trait_change('bar[]')
...     def report_bar_change(self, obj, name, old, new):
...         print(f"Some bar values changed: {old} --> {new}")
...
>>> f = Foo()

all of the following will notify listeners:

>>> f.bar[0] = 1
Some bar values changed: [1] --> [1]
>>> f.bar[:2] = [1, 2]
Some bar values changed: [1, 2] --> [1, 2]

The exception - no structural change:

>>> f.bar[3:] = []
>>>

Checklist

• Tests
• [ ] Update API reference (docs/source/traits_api_reference)
• [ ] Update User manual (docs/source/traits_user_manual)
• [ ] Update type annotation hints in traits-stubs

[codecov-io]

Codecov Report

Merging #1026 into master will increase coverage by 2.67%.
The diff coverage is 97.25%.

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   73.05%   75.73%   +2.67%     
==========================================
  Files          51       54       +3     
  Lines        6514     6511       -3     
  Branches     1309     1280      -29     
==========================================
+ Hits         4759     4931     +172     
+ Misses       1363     1227     -136     
+ Partials      392      353      -39     

Impacted Files	Coverage Δ
traits/api.py	90.32% <ø> (ø)
traits/trait_base.py	68.83% <ø> (+5.25%)	⬆️
traits/trait_dict_object.py	75.44% <ø> (ø)
traits/trait_set_object.py	95.42% <ø> (+29.20%)	⬆️
traits/trait_types.py	72.45% <92.59%> (+0.81%)	⬆️
traits/trait_list_object.py	97.51% <97.69%> (+23.22%)	⬆️
traits/ctrait.py	71.07% <100.00%> (+12.51%)	⬆️
traits/has_traits.py	72.75% <100.00%> (+1.50%)	⬆️
traits/observers/_i_observer.py	100.00% <100.00%> (ø)
traits/observers/_named_trait_observer.py	100.00% <100.00%> (ø)
... and 17 more

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[mdickinson]

LGTM