Add Enum constants for trait kinds, validators and default values

[corranwebster]

This is an attempt to remove as many magic numbers as possible from the traits source code, plus use new Python 3 features to do it.

Possibly should also add some C constants for validator types to ctraits.c.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@13f27de). Click here to learn what that means.
The diff coverage is 90.41%.

@@            Coverage Diff            @@
##             master     #680   +/-   ##
=========================================
  Coverage          ?   70.81%           
=========================================
  Files             ?       45           
  Lines             ?     6342           
  Branches          ?     1291           
=========================================
  Hits              ?     4491           
  Misses            ?     1447           
  Partials          ?      404

Impacted Files	Coverage Δ
traits/api.py	89.28% <100%> (ø)
traits/has_traits.py	70.16% <100%> (ø)
traits/constants.py	100% <100%> (ø)
traits/trait_base.py	53.78% <100%> (ø)
traits/trait_numeric.py	51.97% <100%> (ø)
traits/traits_listener.py	79.08% <100%> (ø)
traits/trait_handlers.py	60.68% <75.86%> (ø)
traits/traits.py	68.91% <77.77%> (ø)
traits/trait_types.py	66.72% <89.13%> (ø)

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[mdickinson]

LGTM; a couple of typos and a suggestion to document the NO_COMPARE, ... constants.

I merged master (badly), fixed the bad merge, and also took the liberty of fixing one remaining use of NO_COMPARE in a test.

[mdickinson]

Will fix merge conflicts shortly.

[mdickinson]

Assigning to me for merge conflict resolution, minor updates, and merge.