Update flyweight pattern

[gyermolenko]

Current example of Flyweight has actually two of them: with overriding __new__ and with metaclass.
I found the one with metaclass especially hard to follow and decided to split-up the examples.

What this PR does:

• move metaclass example to a separate file
• move from printing outputs to doctest style
• add a bunch of comments and a link to Brandon Rhodes blog post about flyweights

[codecov-io]

Codecov Report

Merging #291 into master will increase coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
+ Coverage   45.51%   45.69%   +0.17%     
==========================================
  Files          41       42       +1     
  Lines        1584     1578       -6     
==========================================
  Hits          721      721              
+ Misses        863      857       -6

Impacted Files	Coverage Δ
...tterns/structural/flyweight_with_metaclass__py3.py	0% <0%> (ø)
patterns/structural/flyweight__py3.py	0% <0%> (ø)	⬆️

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