Mypy: fix helper functions

[jstucke]

supersedes #1123 and #1142

• added mypy config to pyproject.toml
• fixed mypy type issues in all modules in helperFunctions

this does not include the pre-commit hook yet (otherwise all other builds would fail after merging this PR)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.23810% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.43%. Comparing base (c93cb37) to head (321dc7c).

Files with missing lines	Patch %	Lines
src/helperFunctions/plugin.py	77.77%	2 Missing ⚠️
src/helperFunctions/fileSystem.py	0.00%	1 Missing ⚠️
src/objects/file.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
- Coverage   92.43%   92.43%   -0.01%     
==========================================
  Files         370      370              
  Lines       20833    20818      -15     
==========================================
- Hits        19258    19243      -15     
  Misses       1575     1575              

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[maringuu]

In an effort to separate commits that actually change codes from commits that only change type hints, I added 41e36ac. Would you mind improving the commit message (at least removing the WIP prefix)?