add more type annotations

[a-detiste]

So I continue adding type annotations, more commits will follow

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (ad40215) 81.26% compared to head (9eb7fee) 81.12%.

Files	Patch %	Lines
duecredit/tests/test_api.py	72.22%	4 Missing and 1 partial ⚠️
duecredit/tests/mod/submod.py	0.00%	3 Missing ⚠️
duecredit/utils.py	50.00%	3 Missing ⚠️
duecredit/tests/test_collector.py	93.93%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   81.26%   81.12%   -0.14%     
==========================================
  Files          47       47              
  Lines        2535     2559      +24     
  Branches      358      362       +4     
==========================================
+ Hits         2060     2076      +16     
- Misses        384      390       +6     
- Partials       91       93       +2     

Flag	Coverage Δ
unittests	81.00% <81.15%> (-0.14%)	⬇️

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