mypy improvement

[Lee-W]

Description

This is part of https://github.com/commitizen-tools/commitizen/issues/1053, which intends to catch errors earlier through a more strict mypy setting.

Checklist

• [x] Add test cases to all the changes you introduce
• [x] Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• [x] Test the changes on the local machine manually
• [x] Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

Additional context

#1053

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.57%. Comparing base (120d514) to head (06eef86). Report is 413 commits behind head on master.

Files	Patch %	Lines
commitizen/commands/check.py	75.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1054      +/-   ##
==========================================
+ Coverage   97.33%   97.57%   +0.23%     
==========================================
  Files          42       55      +13     
  Lines        2104     2518     +414     
==========================================
+ Hits         2048     2457     +409     
- Misses         56       61       +5     

Flag	Coverage Δ
unittests	97.57% <95.00%> (+0.23%)	:arrow_up:

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