feat: adds typing

[kushaldas]

Now we have typing information added to the project and also enabled checking in CI.

Need help: someone please help with those 3 lines of missing coverage. I will try to add something with a static key for testing those lines for codecov.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (1ff2114) to head (f599d96).
⚠️ Report is 13 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   90.17%   91.54%   +1.37%     
==========================================
  Files          15       15              
  Lines        2625     2674      +49     
  Branches       29       27       -2     
==========================================
+ Hits         2367     2448      +81     
+ Misses        255      224      -31     
+ Partials        3        2       -1     

Flag	Coverage Δ
unittests	91.54% <100.00%> (+1.37%)	⬆️

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[MatthiasValvekens]

Thanks for this contribution! Approved in principle, modulo a couple nits.