Separate version specific scripts

[gyermolenko]

@faif
This PR is related to our discussion in #208

#208 (comment)

maybe it's better to split those files to Python 2/3 versions until the Python 2 EOL

There are 4 scripts with metaclasses (chain_of_responsibility, registry, blackboard, flyweight).
I modified 2/4 here to get your feedback. If you are ok with these changes - please let me know. I'll finish work in this or separate PR.

[codecov-io]

Codecov Report

Merging #287 into master will decrease coverage by 3.35%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
- Coverage   70.21%   66.85%   -3.36%     
==========================================
  Files          56       58       +2     
  Lines        2058     2091      +33     
==========================================
- Hits         1445     1398      -47     
- Misses        613      693      +80

Impacted Files	Coverage Δ
tests/test_outputs.py	100% <ø> (ø)	⬆️
...atterns/behavioral/chain_of_responsibility__py2.py	0% <0%> (ø)
patterns/behavioral/registry__py2.py	0% <0%> (ø)
...atterns/behavioral/chain_of_responsibility__py3.py	0% <0%> (ø)
patterns/behavioral/registry__py3.py	0% <0%> (ø)

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