allow pickling / depickling of odict_keys, odict_values, odict_items

[ogrisel]

I broke the original #423 PR. Let me fix it here.

[codecov]

Codecov Report

Merging #429 (c295f5d) into master (685bf0c) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
+ Coverage   91.80%   91.99%   +0.19%     
==========================================
  Files           4        4              
  Lines         671      687      +16     
  Branches      137      141       +4     
==========================================
+ Hits          616      632      +16     
  Misses         34       34              
  Partials       21       21              

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	87.21% <100.00%> (+0.25%)	⬆️
cloudpickle/cloudpickle_fast.py	96.87% <100.00%> (+0.09%)	⬆️

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