Remove py2 compatibility-related stuff

[gyermolenko]

No description provided.

[codecov-io]

Codecov Report

Merging #309 into master will decrease coverage by 0.36%.
The diff coverage is 40.22%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   37.05%   36.68%   -0.37%     
==========================================
  Files          38       38              
  Lines        1325     1341      +16     
==========================================
+ Hits          491      492       +1     
- Misses        834      849      +15

Impacted Files	Coverage Δ
patterns/behavioral/registry.py	0% <ø> (ø)	⬆️
patterns/behavioral/iterator.py	0% <ø> (ø)	⬆️
patterns/behavioral/chain_of_responsibility.py	0% <ø> (ø)	⬆️
patterns/behavioral/publish_subscribe.py	98.18% <ø> (ø)	⬆️
patterns/other/graph_search.py	0% <ø> (ø)	⬆️
patterns/behavioral/strategy.py	0% <ø> (ø)	⬆️
patterns/structural/composite.py	0% <ø> (ø)	⬆️
patterns/behavioral/template.py	0% <ø> (ø)	⬆️
patterns/structural/proxy.py	74.19% <ø> (-0.81%)	⬇️
patterns/dependency_injection.py	0% <0%> (ø)	⬆️
... and 37 more

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