Make some previously-unpickleable trait types pickleable.

[mdickinson]

This PR makes traits of type Instance, Type and UUID pickleable on Python 2.7; since Traits 5.0, these have been pickleable only on Python 3.

This ameliorates (but doesn't really fix) the issues described in #452, and we may still want to look into other solutions to #452. Nevertheless, it seems like a useful minor improvement independent of any other fix applied for #452.

Some remaining awkwardnesses:

• Range traits aren't currently pickleable on Python 2.7, for similar reasons. This can be fixed in a later PR.
• An instance of a HasTraits class with a UUID trait in its __instance_traits__ still can't be roundtripped through a pickle operation, because reconstruction runs into the read-only property of the UUID trait. For example, on this branch:

(traits-test-2.7)bash-3.2$ python
Enthought Deployment Manager -- https://www.enthought.com
Python 2.7.13 |Enthought, Inc. (x86_64)| (default, Mar  2 2017, 08:20:50) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from traits.api import *
>>> class A(HasTraits): foo = UUID()
... 
>>> a = A()
>>> a.on_trait_change(lambda: None, 'foo')  # force existence of an `__instance_trait__` entry
>>> import pickle
>>> pickle.dumps(a)
"ctraits.traits\n__newobj__\np0\n(c__main__\nA\np1\ntp2\nRp3\n(dp4\nS'__traits_version__'\np5\nS'5.1.0.dev33'\np6\nsS'foo'\np7\nccopy_reg\n_reconstructor\np8\n(cuuid\nUUID\np9\nc__builtin__\nobject\np10\nNtp11\nRp12\n(dp13\nS'int'\np14\nL157809889254046028519129337824636172114L\nsbsS'__instance_traits__'\np15\n(dp16\ng7\ng0\n(ctraits.traits\nCTrait\np17\nI0\ntp18\nRp19\n(I0\nI0\nI5\nNI14\nI-1\nI7\n(cuuid\nuuid4\np20\n(tNtp21\nI0\nNNI4\nNg8\n(ctraits.trait_types\nUUID\np22\ng10\nNtp23\nRp24\n(dp25\nS'default_value'\np26\nNsS'_metadata'\np27\n(dp28\nS'type'\np29\nS'trait'\np30\nssb(dp31\ng29\ng30\nstp32\nbssb."
>>> pickle.loads(pickle.dumps(a))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/.edm/envs/traits-test-2.7/lib/python2.7/pickle.py", line 1388, in loads
    return Unpickler(file).load()
  File "/Users/mdickinson/.edm/envs/traits-test-2.7/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/Users/mdickinson/.edm/envs/traits-test-2.7/lib/python2.7/pickle.py", line 1223, in load_build
    setstate(state)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/has_traits.py", line 1567, in __setstate__
    self.trait_set(trait_change_notify=trait_change_notify, **state)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/has_traits.py", line 1672, in trait_set
    setattr(self, name, value)
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 3704, in validate
    "UUID." % (name, class_of(object))
traits.trait_errors.TraitError: The 'foo' trait of an A instance is a read-only UUID.

[mdickinson]

since Traits 5.0, these have been pickleable only on Python 3.

To be clear, these traits have never been pickleable on Python 2, but #373 caused attempts to pickle these traits to be much more common.

[codecov-io]

Codecov Report

Merging #457 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
+ Coverage   65.27%   65.57%   +0.29%     
==========================================
  Files          44       44              
  Lines        7053     7055       +2     
  Branches     1410     1410              
==========================================
+ Hits         4604     4626      +22     
+ Misses       2027     2008      -19     
+ Partials      422      421       -1

Impacted Files	Coverage Δ
traits/trait_types.py	63.99% <100%> (+0.83%)	⬆️
traits/etsconfig/etsconfig.py	63.58% <0%> (+6.17%)	⬆️

---

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 3883302...870e43b. Read the comment docs.

[jwiggins]

We made a UUID trait which can be assigned at initialization time for a customer project. It's as easy as adding a small check to the validate method:

if instance.traits_inited():
    raise TraitError('A UUID trait can only be set at object initialization time!')

[mdickinson]

We don't care about Python 2 any more. Closing this PR.