Add dependabot with monthly schedule

[Kludex]

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[codecov-commenter]

Codecov Report

Merging #39 (de67d6c) into master (60186bd) will not change coverage.
The diff coverage is n/a.

❗ Current head de67d6c differs from pull request most recent head 00a005c. Consider uploading reports for the commit 00a005c to get more accurate results

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   86.20%   86.20%           
=======================================
  Files           7        7           
  Lines         145      145           
=======================================
  Hits          125      125           
  Misses         20       20           

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