refactor: Fix more easy mypy issues.

[bvanelli]

• Numerous fixes to the fit the mypy type checker
• Add mypy workflow check on Github actions
• Rules file still remains ugly with a lot of type-ignores since it was very hard to get the types right without a bigger refactor. They should be tackled in a follow up.

Closes #190

[codecov]

Codecov Report

❌ Patch coverage is 96.69421% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.54%. Comparing base (8bf97c6) to head (a0b67dc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
actual/rules.py	96.80%	3 Missing ⚠️
actual/__init__.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
- Coverage   97.62%   97.54%   -0.08%     
==========================================
  Files          21       21              
  Lines        3238     3257      +19     
==========================================
+ Hits         3161     3177      +16     
- Misses         77       80       +3     

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