mypy improvement

[Lee-W]

Description

This is part of #1053, which intends to catch errors earlier through a more strict mypy setting.

Checklist

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

Expected behavior

Steps to Test This Pull Request

Additional context

#1053

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.58%. Comparing base (120d514) to head (1c70b89).
Report is 479 commits behind head on master.

Files with missing lines	Patch %	Lines
commitizen/cz/customize/customize.py	84.61%	2 Missing ⚠️
commitizen/commands/check.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1054      +/-   ##
==========================================
+ Coverage   97.33%   97.58%   +0.24%     
==========================================
  Files          42       55      +13     
  Lines        2104     2605     +501     
==========================================
+ Hits         2048     2542     +494     
- Misses         56       63       +7     

Flag	Coverage Δ
unittests	97.58% <88.00%> (+0.24%)	⬆️

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[woile]

Lgtm