Updating code to Python 3

[gyermolenko]

Per discussion in #208

This is definitely not every change I wanted to make regarding py2/3.
And I plan to continue related work in separate PRs.

[codecov-io]

Codecov Report

Merging #308 into master will increase coverage by 4.37%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
+ Coverage   32.46%   36.83%   +4.37%     
==========================================
  Files          42       38       -4     
  Lines        1534     1352     -182     
==========================================
  Hits          498      498              
+ Misses       1036      854     -182

Impacted Files	Coverage Δ
patterns/behavioral/registry.py	0% <ø> (ø)
patterns/structural/flyweight_with_metaclass.py	0% <ø> (ø)
patterns/behavioral/chain_of_responsibility.py	0% <ø> (ø)
patterns/other/blackboard.py	0% <ø> (ø)
patterns/structural/flyweight.py	0% <ø> (ø)
patterns/creational/pool.py	50% <0%> (+4.05%)	⬆️

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