Fix date filtering of DWD observations

[gutzbenj]

fixes #356

[codecov]

Codecov Report

Merging #357 (7fb5d8d) into main (bd0d37b) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
+ Coverage   90.12%   90.26%   +0.14%     
==========================================
  Files          53       53              
  Lines        2785     2785              
  Branches      287      287              
==========================================
+ Hits         2510     2514       +4     
+ Misses        217      214       -3     
+ Partials       58       57       -1     

Flag	Coverage Δ
unittests	90.26% <100.00%> (+0.14%)	⬆️

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Impacted Files	Coverage Δ
wetterdienst/dwd/observations/api.py	88.10% <100.00%> (+1.48%)	⬆️

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