Only produce items_changed events on deleting list elements when items are actually removed

[k2bd]

Closes #738, unless there are additional ways of triggering this issue that I was not able to find.

TraitListObject.__delitem__ only emits an event if items are actually removed.

[codecov-io]

Codecov Report

Merging #740 into master will increase coverage by 0.25%.
The diff coverage is 72.87%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
+ Coverage    71.4%   71.66%   +0.25%     
==========================================
  Files          45       51       +6     
  Lines        6288     6355      +67     
  Branches     1282     1283       +1     
==========================================
+ Hits         4490     4554      +64     
- Misses       1395     1404       +9     
+ Partials      403      397       -6

Impacted Files	Coverage Δ
traits/util/clean_strings.py	51.72% <ø> (ø)	⬆️
traits/testing/optional_dependencies.py	94.44% <ø> (ø)	⬆️
traits/adaptation/adaptation_manager.py	98.24% <ø> (ø)	⬆️
traits/etsconfig/etsconfig.py	63.8% <ø> (+0.22%)	⬆️
traits/__init__.py	71.42% <ø> (ø)	⬆️
traits/util/import_symbol.py	100% <ø> (ø)	⬆️
traits/adaptation/adapter.py	100% <ø> (ø)	⬆️
traits/testing/unittest_tools.py	96.03% <ø> (ø)	⬆️
traits/util/async_trait_wait.py	100% <ø> (ø)	⬆️
traits/adaptation/adaptation_offer.py	94.11% <ø> (ø)	⬆️
... and 51 more

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[k2bd]

PR ready for review

[mdickinson]

LGTM