Improve HasTraits introspection with dir()

[midhun-pm]

Fix #925
Includes the names from super().__dir__() in the HasTraits.__dir__'s output.

[codecov-io]

Codecov Report

Merging #927 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
+ Coverage   72.76%   72.95%   +0.18%     
==========================================
  Files          51       51              
  Lines        6474     6456      -18     
  Branches     1302     1301       -1     
==========================================
- Hits         4711     4710       -1     
+ Misses       1367     1347      -20     
- Partials      396      399       +3

Impacted Files	Coverage Δ
traits/has_traits.py	71.24% <100%> (-0.08%)	⬇️
traits/editor_factories.py	81.25% <0%> (-3.13%)	⬇️
traits/adaptation/adaptation_offer.py	93.75% <0%> (-0.37%)	⬇️
traits/trait_set_object.py	66.89% <0%> (-0.23%)	⬇️
traits/trait_types.py	71.07% <0%> (-0.16%)	⬇️
traits/adaptation/adaptation_manager.py	98.19% <0%> (-0.05%)	⬇️
traits/testing/unittest_tools.py	96% <0%> (-0.04%)	⬇️
traits/traits.py	76.92% <0%> (+4.04%)	⬆️
traits/etsconfig/etsconfig.py	63.46% <0%> (+5.79%)	⬆️

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[mdickinson]

LGTM. I'll test this with Envisage before merging (we've had compatibility issues between the fancy dir and Envisage previously).

[mdickinson]

Envisage seems fine: the Envisage test suite runs without error on Envisage master + Traits from this branch. Merging.

[mdickinson]

Aargh. Sorry, @corranwebster: I should have waited for you to confirm that you're happy that your comments have been addressed before merging. Let me know if not, and we can revert or do a follow-up PR as appropriate.