Improve SSAFile type annotations

[RafaelMuijsert]

This patch refactors type annotations in ssafile.py to fit modern PEP standards.

Furthermore, moving type hints to class-level annotations helps basedpyright deduce types, which it was previously not able to do.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.22%. Comparing base (a9ef790) to head (43b7d09).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   92.20%   92.22%   +0.02%     
==========================================
  Files          40       40              
  Lines        2962     2970       +8     
  Branches      301      301              
==========================================
+ Hits         2731     2739       +8     
  Misses        151      151              
  Partials       80       80              

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