Add more doctests

[gyermolenko]

No description provided.

[codecov-io]

Codecov Report

Merging #316 into master will decrease coverage by 5.11%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
- Coverage   36.83%   31.71%   -5.12%     
==========================================
  Files          38       38              
  Lines        1276     1258      -18     
==========================================
- Hits          470      399      -71     
- Misses        806      859      +53

Impacted Files	Coverage Δ
patterns/behavioral/specification.py	0% <0%> (-83.06%)	⬇️
patterns/behavioral/state.py	94.59% <0%> (-3.03%)	⬇️
patterns/behavioral/publish_subscribe.py	91.17% <0%> (-6.98%)	⬇️
patterns/creational/borg.py	76.92% <50%> (+40.92%)	⬆️
patterns/structural/mvc.py	0% <0%> (ø)	⬆️
patterns/structural/flyweight_with_metaclass.py	0% <0%> (ø)	⬆️
patterns/behavioral/catalog.py	0% <0%> (ø)	⬆️
patterns/behavioral/memento.py	0% <0%> (ø)	⬆️
... and 5 more

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