Fix incorrect event index for slice with stop < start

[mdickinson]

Fixes #994.

Checklist

• Tests

[codecov-io]

Codecov Report

Merging #999 into master will increase coverage by 1.04%.
The diff coverage is 96.21%.

@@            Coverage Diff             @@
##           master     #999      +/-   ##
==========================================
+ Coverage   73.05%   74.10%   +1.04%     
==========================================
  Files          51       51              
  Lines        6514     6453      -61     
  Branches     1309     1277      -32     
==========================================
+ Hits         4759     4782      +23     
+ Misses       1363     1305      -58     
+ Partials      392      366      -26     

Impacted Files	Coverage Δ
traits/trait_base.py	62.98% <ø> (-0.60%)	⬇️
traits/trait_types.py	71.72% <88.88%> (+0.07%)	⬆️
traits/trait_list_object.py	97.48% <97.68%> (+23.20%)	⬆️
traits/ctrait.py	61.98% <100.00%> (+3.42%)	⬆️
traits/has_traits.py	71.31% <100.00%> (+0.07%)	⬆️
traits/trait_handlers.py	61.11% <100.00%> (ø)
traits/trait_type.py	77.21% <100.00%> (-1.10%)	⬇️
traits/traits.py	76.63% <100.00%> (+3.76%)	⬆️
traits/editor_factories.py	79.59% <0.00%> (-9.19%)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b0c013d...36f4606. Read the comment docs.

[kitchoi]

Thanks! It is interesting to see how this bug-for-bug implementation changes face over time. For reference, the history started here: cc83835
then this: 010b774

Now the if key.start is None is moved to _normalize_slice, that line is back to how it was again.

[kitchoi]

LGTM

[mdickinson]

this bug-for-bug implementation changes face over time

Agreed. I don't think we actually have bugs in this part of TraitListObject any more, and there's no need for bug-for-bug compatibility code at this point. I actually want to change TraitList to match TraitListObject here. (See comments in #997)