Add TraitChangeEvent object for wrapping CTrait change

[kitchoi]

This PR adds the TraitChangeEvent object for wrapping CTrait change events.

This event object will be given to the user's handler registered using observe.

This event object will also be used by the NamedTraitObserver in #1069, the "FilteredTraitObserver" for observing traits using a filter, and the (internal-use-only) observer for trait_added events.

Checklist

• Tests
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual) WIP in Add user manual for observe #1060
• Update type annotation hints in traits-stubs

[codecov-io]

Codecov Report

Merging #1070 into master will decrease coverage by 3.33%.
The diff coverage is 94.84%.

@@            Coverage Diff             @@
##           master    #1070      +/-   ##
==========================================
- Coverage   76.15%   72.82%   -3.34%     
==========================================
  Files          54       65      +11     
  Lines        6493     8047    +1554     
  Branches     1263     1538     +275     
==========================================
+ Hits         4945     5860     +915     
- Misses       1205     1808     +603     
- Partials      343      379      +36     

Impacted Files	Coverage Δ
traits/api.py	100.00% <ø> (+9.67%)	⬆️
traits/base_trait_handler.py	61.76% <ø> (ø)
traits/ctrait.py	71.07% <ø> (ø)
traits/has_traits.py	72.40% <ø> (-0.37%)	⬇️
traits/observers/_i_notifier.py	0.00% <0.00%> (ø)
traits/observers/_i_observable.py	0.00% <0.00%> (ø)
traits/observers/events.py	0.00% <0.00%> (ø)
traits/traits.py	75.10% <ø> (-2.45%)	⬇️
traits/util/resource.py	15.25% <ø> (ø)
traits/trait_types.py	72.15% <80.00%> (-0.31%)	⬇️
... and 29 more

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[ievacerny]

LGTM

[kitchoi]

Thank you! Merging this and I can merge master back to #1069