Revert enthought/traits#373: instance traits are no longer pickleable

[mdickinson]

#373 made instance traits (not Instance traits![*]) pickleable. While the aim was a good one, the change introduced a variety of unintended consequences and breakages, so I'm reverting it here. We may be able to do it again properly one day.

The main issues are:

1. Pickling instance traits requires that the corresponding TraitType objects are pickleable (whereas for class-level traits, we only need to pickle the trait values). Many TraitType objects currently aren't pickleable, especially in Python 2. That could certainly be fixed: see Make some previously-unpickleable trait types pickleable. #457 for some work in that direction, but there's much more to do.

2. Even in the absence of use of add_trait (which is relatively rare in most Traits projects), the _instance_traits dictionary has entries added to it whenever a listener is attached to a class trait on a HasTraits object: the instance trait acts as the place to register listeners that are specific to that instance. This meant that Resolve pickling and deepcopying bug with dynamically added traits #373 resulted in attempts to pickle various (as it turns out unpickleable) class-level traits, breaking existing code. This again, could be fixed, at the cost of some work and some restructuring.

Point 2 is what causes the unexpected breakage of previously working code.

I'll re-open the various issues that were closed by #373.

[*] Not "Instance" traits! This issue is about traits added directly to an object (e.g., with HasTraits.add_trait), as opposed to traits declared at class level.

Closes #452

[mdickinson]

And in case, like me, you're wondering why #373 added 35 lines and this PR removes 37, the blackening of the codebase in #432 introduced two extra blank lines following local imports.

[codecov-io]

Codecov Report

Merging #462 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #462      +/-   ##
==========================================
- Coverage   65.29%   65.26%   -0.03%     
==========================================
  Files          44       44              
  Lines        7065     7057       -8     
  Branches     1413     1411       -2     
==========================================
- Hits         4613     4606       -7     
+ Misses       2029     2025       -4     
- Partials      423      426       +3

Impacted Files	Coverage Δ
traits/has_traits.py	67.37% <ø> (-0.44%)	⬇️
traits/trait_base.py	55.79% <0%> (-2.18%)	⬇️
traits/traits.py	60% <0%> (-0.2%)	⬇️
traits/trait_handlers.py	63.07% <0%> (-0.14%)	⬇️
traits/etsconfig/etsconfig.py	63.58% <0%> (+6.17%)	⬆️

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[corranwebster]

Looks good. The only line in the original PR that is not removed is an import of TraitError which appears to be used by other tests and so is still needed.

[mdickinson]

Thanks for the review!