test: enable mypy strict mode

[jooola]

• Increase type coverage
• Fix invalid types

Closes #479

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.78%. Comparing base (3df9e69) to head (17c6e19).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hcloud/rdns/domain.py	0.00%	5 Missing ⚠️
hcloud/rdns/__init__.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
- Coverage   96.94%   96.78%   -0.17%     
==========================================
  Files          74       76       +2     
  Lines        3993     4038      +45     
==========================================
+ Hits         3871     3908      +37     
- Misses        122      130       +8     

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