Add mypy to CI and fix issues in the code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (201ad38) 100.00% compared to head (77de254) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #76 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 96 99 +3 Branches 14 15 +1 ========================================= + Hits 96 99 +3 ```

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