Rename Factory method to Factory

[gyermolenko]

I would agree with the creator of #177
As I understand it - Factory method pattern is about class methods.
Such methods should be implemented or overridden by child classes (in languages where there is no other choice and where classes and functions are not first-class citizens)

refs:
https://python-patterns.guide/gang-of-four/factory-method/
https://en.wikipedia.org/wiki/Factory_method_pattern

I would also list Factory method along with Singleton in anti-patterns section (things to avoid).

Changes from this PR:

• fix: this script is now python2 compatible (it wasn't)
• rename factory_method.py to factory.py
• update to use doctests
• rename some variables to be more readable

P.S.: congratulations on 20k stars! 🌟

[codecov-io]

Codecov Report

Merging #294 into master will decrease coverage by 0.87%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   45.51%   44.64%   -0.88%     
==========================================
  Files          41       41              
  Lines        1584     1586       +2     
==========================================
- Hits          721      708      -13     
- Misses        863      878      +15

Impacted Files	Coverage Δ
patterns/creational/factory.py	0% <0%> (ø)

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