Fix mypy violations

[bartfeenstra]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.22%. Comparing base (5cc02f2) to head (94fc6b2).
Report is 1 commits behind head on 0.4.x.

Files with missing lines	Patch %	Lines
betty/date.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            0.4.x    #2260   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files         365      365           
  Lines       23209    23212    +3     
  Branches     1071     1071           
=======================================
+ Hits        22797    22800    +3     
  Misses        264      264           
  Partials      148      148           

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