Create a metaclass for Singleton's and remove pysingleton

[alexandrehassan]

I don't know if this is where you want the singleton Meta class to be (or the tests). This completely removes the pysingleton library. Closes #471 .

Added quick tests to prove that it replaces the old singleton.

(it was easier to create a new branch (old MR #492)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.53%. Comparing base (49dbe35) to head (10fdc60).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #494      +/-   ##
==========================================
+ Coverage   87.50%   87.53%   +0.03%     
==========================================
  Files          38       38              
  Lines        2329     2335       +6     
==========================================
+ Hits         2038     2044       +6     
  Misses        291      291              

Flag	Coverage Δ
unittests	87.53% <100.00%> (+0.03%)	⬆️

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