FIX Cast to string _flatten_dict

[marload]

What does this PR do?

Fixes #5311 <- this links related issue to this PR

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[codecov]

Codecov Report

Merging #5354 (ab5307b) into master (ec0fb7a) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5354   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         134     134           
  Lines        9992    9993    +1     
======================================
+ Hits         9309    9310    +1     
  Misses        683     683           

[s-rog]

Could you also add more types to the test besides int since it's Any now?

[awaelchli]

great