Implement reduce for objects derived from abstract decorators

[KristianHolsheimer]

This PR fixes #367

[codecov]

Codecov Report

Merging #369 into master will decrease coverage by 32.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #369       +/-   ##
===========================================
- Coverage   92.95%   60.88%   -32.07%     
===========================================
  Files           2        2               
  Lines         809      813        +4     
  Branches      164      164               
===========================================
- Hits          752      495      -257     
- Misses         29      305      +276     
+ Partials       28       13       -15     

Impacted Files	Coverage Δ
cloudpickle/cloudpickle_fast.py	95.79% <100.00%> (+0.07%)	⬆️
cloudpickle/cloudpickle.py	46.43% <0.00%> (-45.40%)	⬇️

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[KristianHolsheimer]

I don't have time to work on this to figure out what goes wrong with python versions < 3.8.

I'll abandon this PR, but I'll leave the issue #369