Raise TraitNotificationError on trailing comma in on_trait_change spec.

[midhun-pm]

Fixes #406

Modifies the ListenerParser which parses the name for the on_trait_changedecorator to check for a trailing ,.
Raises a TraitNotificationError error if trailing comma is present in the trimmed string.

[codecov-io]

Codecov Report

Merging #926 into master will increase coverage by 0.21%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   72.90%   73.12%   +0.21%     
==========================================
  Files          51       51              
  Lines        6456     6460       +4     
  Branches     1301     1302       +1     
==========================================
+ Hits         4707     4724      +17     
+ Misses       1357     1341      -16     
- Partials      392      395       +3     

Impacted Files	Coverage Δ
traits/editor_factories.py	79.59% <0.00%> (-4.79%)	⬇️
traits/trait_base.py	62.96% <ø> (-0.46%)	⬇️
traits/constants.py	100.00% <100.00%> (ø)
traits/trait_handlers.py	61.11% <100.00%> (+0.08%)	⬆️
traits/traits_listener.py	81.64% <100.00%> (+0.07%)	⬆️
traits/trait_types.py	71.07% <0.00%> (-0.16%)	⬇️
traits/traits.py	76.92% <0.00%> (+4.04%)	⬆️
traits/etsconfig/etsconfig.py	63.46% <0.00%> (+6.41%)	⬆️

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[kitchoi]

I am sorry that #921 has just been merged. It added a test that will need to be changed as part of this PR as well.

[mdickinson]

Please could you address @kitchoi's comments?

[midhun-pm]

Addressed PR comments. Sorry for the delay, lost track of this one.

[kitchoi]

LGTM!

[mdickinson]

The macOS Python 3.6 build seems flaky today. I've restarted it (again).

[mdickinson]

Ah, sorry; forgot that my own previous review would block merging.

[mdickinson]

LGTM