TYP: improve type-correctness

[neutrinoceros]

• TYP: switch to GHA for mypy instead of pre-commit
• TYP: fix some typing errors

[codecov]

Codecov Report

Merging #93 (cce56ba) into main (73c1668) will not change coverage.
The diff coverage is 100.00%.

❗ Current head cce56ba differs from pull request most recent head 2984158. Consider uploading reports for the commit 2984158 to get more accurate results

@@            Coverage Diff            @@
##              main       #93   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          638       637    -1     
=========================================
- Hits           638       637    -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
inifix/_typing.py	100.00% <100.00%> (ø)

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