MNT: Create dependabot.yml for workflow auto updates

[pllim]

As recommended by https://scientific-python.org/specs/spec-0008/#pin-github-actions-release-workflows-to-their-full-release-commit-shas , this PR adds a dependabot.yml file to enable future automatic updates of GitHub Actions workflow(s) in this repository.

This is an automated update made by the batchpr tool :robot: - feel free to close if it doesn't look good! You can report issues to @pllim.

:ghost:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.28%. Comparing base (eb9d316) to head (1cbadc3). Report is 78 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
- Coverage   87.28%   87.28%   -0.01%     
==========================================
  Files          22       21       -1     
  Lines        3821     3813       -8     
==========================================
- Hits         3335     3328       -7     
+ Misses        486      485       -1     

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