Implement --substitute

[PapyChacal]

Enables generation of variable substituions for blocks and variables, as opposed to blind anonymization.
This keeps the test robust against naming changes, while checking the right things are used in the right places 🎉

Also, let Python 3.7 go in peace. It's reached end-of-life and has some specific formatting check error for some reason 🕊️

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.72093% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.66%. Comparing base (fce0f4e) to head (74c700a).

Files	Patch %	Lines
filecheckize/main.py	83.72%	6 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   97.72%   90.66%   -7.07%     
==========================================
  Files           1        1              
  Lines          44       75      +31     
  Branches        9       15       +6     
==========================================
+ Hits           43       68      +25     
- Misses          0        6       +6     
  Partials        1        1              

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