Fix flake8

[gyermolenko]

As it turns out flake8 *py command checks only files by that mask in current folder, w/o going into subfolders. Fix required to also fix existing warnings.

In addition to that:

• added # noqa to append_output.sh because multiline output string is also checked by flake8 (which is weird but it is current behavior discussed e.g. here)
• suppressed one pytest warning regarding example TestRunner class from facade
• modified setup.py to correspond to changes in #270 (+py37)

[codecov-io]

Codecov Report

Merging #273 into master will decrease coverage by 0.02%. The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   69.85%   69.82%   -0.03%     
==========================================
  Files          62       62              
  Lines        2289     2287       -2     
==========================================
- Hits         1599     1597       -2     
  Misses        690      690

Impacted Files	Coverage Δ
patterns/behavioral/specification.py	83.33% <ø> (ø)	:arrow_up:
patterns/behavioral/template.py	96.66% <ø> (ø)	:arrow_up:
patterns/behavioral/state.py	97.72% <ø> (ø)	:arrow_up:
patterns/fundamental/delegation_pattern.py	0% <ø> (ø)	:arrow_up:
patterns/behavioral/command.py	96.96% <ø> (ø)	:arrow_up:
setup.py	0% <ø> (ø)	:arrow_up:
patterns/behavioral/memento.py	0% <0%> (ø)	:arrow_up:
patterns/structural/flyweight.py	38.33% <0%> (ø)	:arrow_up:
patterns/behavioral/iterator.py	94.44% <100%> (-0.56%)	:arrow_down:

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